DO NOT MERGE - WIP

This is a PR-to-the-PR-to-the-PR for architectural issues.

Ok, I've got a really messy version of the content negotiation stuff up in a PR (to the PR, to the PR,...). The main purpose is to create and register the single-record formatters (i.e. dcat, etc...):

1. On startup, Generic-GN5 app is run, and WebConfig will be executed. This is where spring content negotiation is setup.
2. WebConfig registers two things - the list of acceptable MIME types and their "short name". This is used in the ?f=json and ?f=application/json.
3. WebConfig hands off to simple MessageWriterUtil class. This gets implementations of IContentNegotiationInitializable from any of the modules.
4. each of these are initialized and creates a set IControllerResultFormatter. In this case the FormatterApi is queried and 4 classes are created - one for each of the 4 MimeTypes that the formatter produces (xml, json, xml+rdf, geojson). These IControllerResultFormatter are profile-aware formatters that use to FormatterApi to do the actual work.
5. These 4 are registered as spring MessageConverters in the normal spring content negotiation.

As mentioned before, when a request for a single-record comes in:

1. spring routes it to the correct controller
2. the controller does very little, but returns a OgcApiRecordsSingleRecordResponse, which is a very small object that contains the catalogId, recordId, and user-requested profiles.
3. This is the handed off to the spring content negotiation system (which looks for things like `?f=application/xml+rdr)
4. Spring will look for a messagewriter that can handle application/xml+rdr for the OgcApiRecordsSingleRecordResponse (this is one of the profile-aware ones we registered).
5. spring will hand the OgcApiRecordsSingleRecordResponse off to our profile-aware FormatterApi setup for application/xml+rdr
6. our formatter will do profile-negotiation (simple) and then hand off to the FormatterApi (knowing both mimetype and official profile name)

WHEW

The main thing is that the content negotation system is pluggable - any app/module can supply a set of IContentNegotiationInitializable that will register everything in the content negotation system.

Key things to look at

1. MessageWriterUtil - doesn't do very much, it just finds IContentNegotiationInitializable and calls #initialize() on them.
2. WebConfig - this is where the MimeTypes (for ?f=...) are setup and message writers (created by MessageWriterUtil) are registered
3. SingleRecordFormatterApiContentNegotiationInitializable - this will create the 4 (one for each of the 4 mime types). These implement profile-aware spring MessageConverters for the OgcApiRecordsSingleRecordResponse
4. OgcApiRecordsSingleRecordResponse - this is the tiny class generated by the controller for a single record
5. OgcApiRecordsSingleRecordResponseFormatter actually does the formatting

PR Title

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• Bug fix
• New feature
• Documentation update
• Refactoring

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Checklist

• Code is formatted and linted
• All tests pass locally and in CI
• Tests updated (if applicable)
• Documentation updated (if applicable)