Refactor of request params parsing process

[numbata]

This pull request splits the request parameter handling into separate parser classes for headers, routes, and bodies. This means the code is more organized and easier to work with, making life easier for contributors and maintainers. It's a step toward adding new features-like contract-based parameter parsing-without having to tangle everything up in one place.

By separating concerns, developers can more easily customize or extend the way parameters are handled. In the future, we will be able to add stricter validation or additional checks simply by creating or updating a parser class. Overall, these changes should keep things cleaner, pave the way for stronger parameter validation, and help the project evolve smoothly.

In two words: my main goal here is to be able to add contract-based request params parser as one of the next steps.

[numbata]

Some changes extracted into #948.