Signposting: use distinct MIME types for HTTP Allow and "is described by" attribute

Overview of the Feature Request

Currently, the Signposting "is described by" links carry rather generic mime types (application/xml, application/json, etc).

It would be good to use a specific MIME Type where available, both in Signposting links and requests to the metadata export API.

What kind of user is the feature intended for?
Users, Curators, ...

What inspired the request?

Looking at Zenodo, this is their Signposting for some random record:

Compare this to our links:

Here we see:

1. The Zenodo links are way shorter because they enable using the specific MIME Type in HTTP Allow to retrieve the format.
2. The MIME-Types from us are less telling what metadata awaits you. Someone looking for say, DataCite or CodeMeta will have a hard time to find the right link. This makes this not really machine actionable - a user can tell from the "exporter" query parameter which format can be expected.

What existing behavior do you want changed?

1. According to the Signposting docs, we should use more specific MIME Types where possible.
2. (Optional): enable using these in API requests from HTTP Allow

Any brand new behavior do you want to add to Dataverse?
Not really brand new.

Any open or closed issues related to this feature request?
gdcc/exporter-croissant#7
#11045
#11151

Are you thinking about creating a pull request for this feature?
Maybe later. Let's discuss first. It would certainly help with harvesting / reporting and machine actionability.

Start by locating the Signposting link generation and metadata export API entry points, then compare how current generic MIME types are selected. Review the Signposting documentation and related issues #11045 and #11151. Done means using specific MIME types where available, with HTTP Allow support treated as the optional follow-up.