[utoipa-gen] MediaTypeAttr, Encoding: Parse encodings

[jsoo1]

Though only in a limited fashion - notable fields are still not
implemented.

Though this does let you do:

   request_body(
       content(...)
       encoding(
           ("field_name", (
               content_type = "application/json",
               explode = true,
               allow_reserved = true,
           ))
       )
   )

Partially addresses #1087

[juhaku]

Looks good, it is missing CHANGELOG.md entry for the addition. Perhaps under Unreleased and Added section by placing newest to top.

[juhaku]

Super, thanks for putting the effort 🙂