Update Handling of File and Bytes for consistency across Single Part and MultiPart bodies

[markcowl]

• Add @mediaTypeHint decorator that specifies the default content-type for scalar types when no content-type is present
• Apply @mediaTypeHint("Application/octet-stream") to bytes scalar and similar hints ('text/plain') to other scalars as appropriate
• Simplify HttpOperation logic to use the mediaTypeHint instead of special processing for bytes
• Treat Http.File the same in multipart and single part bodies
  • contentType is populated from the message or message part content-type header
  • filename is populated from the content-disposition header
  • treatment of content depends on content-type header
  • Http.File in the context of an enclosing object is treated as a json object
• Document requirements for emitters