Support proxy in OpenSearch source

[pablosnt]

Is your feature request related to a problem? Please describe.
I have a pipeline that reads from an OpenSearch index and writes in another one. That OpenSearch server is behind a proxy, so I need to configure it in the source and the sink

Describe the solution you'd like
I think the best solution is adding a proxy to the opensearch source:

source:
  opensearch:
    ...
    proxy: https://myproxy:8080

Or within the connection field:

source:
  opensearch:
    ...
    connection:
      proxy: https://myproxy:8080

Describe alternatives you've considered (Optional)

• proxy field in the opensearch source, or in the connection subfield
• proxy field in the data-prepper-config.yaml
• Environment variable

Additional context
The proxy configuration is already supported for the opensearch sink. It was reported in #300 and implemented in #479

Thank you very much in advance!
I know how hard is maintaining an open source project, so if you need help, I can try to implement this