Add body type getter in Convert trait

[ionut-arm]

Request and response bodies are converted to and from raw byte format using code specific to each body type that we support. In order to have modularity, though, converters are handled through a Convert trait. However, in order to correctly compose requests and responses, you explicitly need to know the BodyType that you're using.

Because the components doing the body conversion usually also create the request or response header (be it either in the service or in the client), it makes sense to be able to retrieve the BodyType straight from the converter, minimizing the risk of the two (body encoding type and encoding type identifier in the header) becoming inconsistent.

[hug-dev]

That's a good point! It will also simplify the BackEndHandler in the service to not duplicate that information.