OAS 3.2.0 multipart media support

[mattpolzin]

Additions to support multipart media types

• New itemSchema field, for the schema that describes each of the items in a sequential media type.
• New fields prefixEncoding and itemEncoding can be used instead of encoding for multipart media types.
• The specification also contains examples of sequential JSON and Server-Sent events that show these fields in use.

[mattpolzin]

Going to roll #441 into this issue.

• application/jsonl, application/json-seq media types

See the media types registry is published to give more context for each of the media types.