Allow passing dict[type, str] to RequestBody()

[rafalkrupinski]

Currently RequstBody follows OpenAPI responses.content map of media-type to python type (schema in OpenAPI).

This must be kept for compatibility with OpenAPI, as lapidary-render will output the map with just schemas replaced with python types.

Additionally a simplified style should be accepted:
a map of python type to concrete media-type (not media-range).

RequestBody({
    ModelType: 'application/json',
    OtherType: 'application/json',
    bytes: 'application/octet-stream',
})

This would be more intuitive to the user, and would unambiguously determine the value of Content-Type header.

Also, the OpenAPI style allows a python type to be mapped to multiple media types, making it necessary to require a parameter for the content-type header.