OpenAPI 3.2 support

[ahx]

What do we have to do to support OAS 3.2? I am not up to date with the newly added features.

• 3.2 support in json_schemer: Add support for OpenAPI 3.2.0 davishmcclurg/json_schemer#228

Here are some things from the release page I think we need to add support for:

1. discriminator and the new field defaultMapping – Does that just work?
2. "Additions to support multipart media types"
   2.1 New itemSchema field
   2.2 New fields prefixEncoding and itemEncoding
3. Support for additional HTTP methods – Under an additionalOperations entry in a Path, use any other methods not listed as keys using the correct capitalization, e.g. LINK. Do NOT add HEAD under this, use the existing sibling head.
4. Support querystring in parameter locations: https://spec.openapis.org/oas/latest.html#parameter-locations