Add @ConsumseJson to contents API. (#999)

### Motivation:
- When a client sends a request with a JSON body but does not configure the `content-type` in header, Central Dogma returns the response`: {"exception":"java.lang.IllegalArgumentException","message":"No suitable request converter found for a @RequestObject 'CommitMessageDto'"}`. This response is quite ambiguous, making it difficult for the client to understand the problem.


### Modifications:
- Add `@ConsumesJson` to each method that uses `ChangesRequestConverter.class`. 

### FYI
1. If `@RequestConverter` is declared on method signatures, Armeria will add an object Resolver. ([link](https://github.com/line/armeria/blob/e2b298dd2f54243e8801540f13e90166f75578b5/core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotatedValueResolver.java#L501-L508))
2. Then, when Armeria receives a request from client, Armeria will try to resolve the request.  ([link](https://github.com/line/armeria/blob/e2b298dd2f54243e8801540f13e90166f75578b5/core/src/main/java/com/linecorp/armeria/internal/server/annotation/AnnotatedValueResolver.java#L829-L862))
3. `ChangesRequestConverter` tries to parse body from request. Actually, `ChangeRequestConverter` delegates to `JacksonRequestConvertFunction`.
4. `JacksonRequestConverter` validates contents type. ([here](https://github.com/line/armeria/blob/e2b298dd2f54243e8801540f13e90166f75578b5/core/src/main/java/com/linecorp/armeria/server/annotation/JacksonRequestConverterFunction.java#L98-L106))
5. If there is no `content-type` or `content-type` is not `application/json`, `JacksonRequestConverter` will call `RequestConverterFunction.fallthrough()`. 

With this flow, client will receive response `: {"exception":"java.lang.IllegalArgumentException","message":"No suitable request converter found for a @RequestObject 'CommitMessageDto'"}`.

If `@ConsumsJson` is delcared to each method having `ChangesRequestConverter.class` on their method signature, Armeria will not try to resolve request without header `Content-Type: application/json`.


### Result:
- Client will receive 415 response.
```sh
$ curl -X POST localhost:8080/my-post \
-d '{"name": "John Doe", "email": "[email protected]"}'
>>
Status: 415
Description: Unsupported Media Type
```
- Closes #987.

server/src/main/java/com/linecorp/centraldogma/server/internal/api/ContentServiceV1.java

@@ -51,6 +51,7 @@
 import com.linecorp.armeria.common.HttpStatus;
 import com.linecorp.armeria.common.util.Exceptions;
 import com.linecorp.armeria.server.ServiceRequestContext;
+import com.linecorp.armeria.server.annotation.ConsumesJson;
 import com.linecorp.armeria.server.annotation.Default;
 import com.linecorp.armeria.server.annotation.Get;
 import com.linecorp.armeria.server.annotation.Param;
@@ -187,6 +188,7 @@ private static String normalizePath(String path) {
      * <p>Pushes a commit.
      */
     @Post("/projects/{projectName}/repos/{repoName}/contents")
+    @ConsumesJson
     @RequiresWritePermission
     public CompletableFuture<PushResultDto> push(
             ServiceRequestContext ctx,
@@ -226,6 +228,7 @@ private CompletableFuture<Revision> push(long commitTimeMills, Author author, Re
      * <p>Previews the actual changes which will be resulted by the given changes.
      */
     @Post("/projects/{projectName}/repos/{repoName}/preview")
+    @ConsumesJson
     public CompletableFuture<Iterable<ChangeDto<?>>> preview(
             ServiceRequestContext ctx,
             @Param @Default("-1") String revision,

server/src/test/java/com/linecorp/centraldogma/server/internal/api/ContentServiceV1Test.java

@@ -31,6 +31,8 @@
 import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.RegisterExtension;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.CsvSource;
 
 import com.fasterxml.jackson.databind.JsonNode;
 
@@ -407,6 +409,39 @@ void getFile() {
             assertThatJson(actualJson).isEqualTo(expectedJson);
         }
 
+        @ParameterizedTest
+        @CsvSource({
+                "application/json, 200",
+                "text/plain, 415",
+                "text/xml, 415",
+                "application/xml, 415",
+                "application/x-www-form-urlencoded, 415"
+        })
+        void getFileWithVariusContentType(String mediaType, int expectedStatusCode) {
+            // Given:
+            final WebClient client = dogma.httpClient();
+            final String body =
+                    '{' +
+                    "   \"path\" : \"/foo.json\"," +
+                    "   \"type\" : \"UPSERT_JSON\"," +
+                    "   \"content\" : {\"a\": \"bar\"}," +
+                    "   \"commitMessage\" : {" +
+                    "       \"summary\" : \"Add foo.json\"," +
+                    "       \"detail\": \"Add because we need it.\"," +
+                    "       \"markup\": \"PLAINTEXT\"" +
+                    "   }" +
+                    '}';
+
+            final RequestHeaders headers = RequestHeaders.of(HttpMethod.POST, CONTENTS_PREFIX,
+                                                             HttpHeaderNames.CONTENT_TYPE, mediaType);
+
+            // When:
+            final AggregatedHttpResponse aRes = client.execute(headers, body).aggregate().join();
+
+            // Then:
+            assertThatJson(aRes.headers().status().code()).isEqualTo(expectedStatusCode);
+        }
+
         @Test
         void getFileWithJsonPath() {
             final WebClient client = dogma.httpClient();