[feature]: support various content types for response

[archerzz]

Clear and concise description of the problem

Right now, we hard code JSON as the response content type, which is wrong:

typespec/packages/http-client-csharp/emitter/src/lib/operation-converter.ts

Lines 254 to 256 in d9faa2a

	BodyType: r.type ? fromSdkType(r.type, sdkContext, modelMap, enumMap) : undefined,
	BodyMediaType: BodyMediaType.Json,
	Headers: fromSdkServiceResponseHeaders(r.headers, sdkContext, modelMap, enumMap),

See an issue reported in #4208 (comment)

We should have support for various kinds of content types. That is:

• replace hard coded media type with the one determined by parsing the operation response
• dump BodyMediaType enum and corresponding reference, we should support a wide range of media types

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[m-nash]

MediaType/StringBody/getAsText is an example of the test that is failing here.

[JoshLove-msft]

@archerzz I believe this is related to the discussion from the sync yesterday regarding how content types should work for both request/response in MGC.

[JoshLove-msft]

We should not be hardcoding anything related to media type. This should come from the compiler only.

[JoshLove-msft]

This has been fixed - we no longer hardcode JSON.