Add support for multipart HTTP request bodies

[pdwilson12]

Bug/issue #, if applicable: rdar://106389324

Summary

Introduce new "HTTPBodyParameter(s)" tags for ListItemExtractor to identify documentation for the parameters that make up a multipart (or URL-encoded) form. These are then merged with symbol graph data wherein each parameter is related to the owning HTTPBody entity via a memberOf or optionalMemberOf relationship.

Dependencies

None.

Testing

Use as a starting point the HTTPRequests.docc fixture, which contains a "CreateArtist.md" file to document the artist and userName body parameters defined in the symbol graph.

Steps:

1. Build docc and preview the fixture.
2. Modify the CreateArtist.md file to add/remove HTTPBodyParameter entries and make sure changes get reflected in the rendered page.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[pdwilson12]

@swift-ci please test

[QuietMisdreavus]

Looks good!

[pdwilson12]

@swift-ci please test

[pdwilson12]

@swift-ci please test