[Core, Rust Server] Support JSON query parameters

Created by: richardwhiuk

This adds support to Rust Server for query parameters encoded in JSON - e.g.

        - name: list-of-strings
          in: query
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/StringObject'

There's a small change to the core code to record the content type, and then there's an update to the Rust Server code to support JSON decoding.

Note, no attempt is made to support anything other than JSON query parameters - you can in theory specify an arbitrary mime type here.

This change was contributed by @mirw and reviewed by myself.

Rust Server Technical Committee

• @frol
• @farcaller
• @bjgill
• @paladinzh

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