Support the multipart content type

[czechboy0]

https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md#special-considerations-for-multipart-content

Let's do a proposal first, as there are a few ways the generated code could go here.

[czechboy0]

This one needs a bit of design, but should be possible.

A simple idea:

• generate a struct for the whole multipart payload
• each property is one content type + payload, decoded as we'd normally decode it if it were a full body
• this struct would not itself be Codable, as we'd generate custom code to decode/encode each of the properties, just like we do for type-safe properties for each operation

[czechboy0]

Additional reading: https://datatracker.ietf.org/doc/html/rfc7578

[czechboy0]

There are also headers associated with each part, not just content type + payload: https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.1.0.md#encoding-object-example

[czechboy0]

Can also be represented as an array property at the top level, at which point it could lend itself well to being a stream of values?