Support multiple request body content types

[czechboy0]

Similar to #6 but for request content.

Should be simpler, as it doesn't require the integration with the Accept header. It'll just be an extension of the existing N=1 media types into N>1. The generated type is already an enum though, so shouldn't require structural changes, just handling all the supported media types instead of picking the first "best" one (like the generator does today).

[czechboy0]

Landed in main under the multipleContentTypes feature flag.

[czechboy0]

Now released in 0.1.7 under the multipleContentTypes feature flag.