Make Croissant built-in to Dataverse (no longer an external exporter)

[pdurbin]

What this PR does / why we need it:

For users, Croissant is increasingly popular and less experimental so it makes sense to enable support for it out of the box in Dataverse, which is what #11254 is about.

For developers, a series of planned changes to Croissant means that it will be easier to maintain the code as part of the core rather than as an external exporter. Once this pull request is merged, it would make sense to work on the following issues in the order below:

• Put a smaller version of Croissant into the head of HTML pages #12123
• Croissant 1.1 #12014
• Explore inside out Croissant (add feature to Dataverse) #12124

Which issue(s) this PR closes:

• Closes enable the Croissant exporter by default (move code to main repo) #11254

Special notes for your reviewer:

The code is equivalent to version 0.1.6 of the external exporter.

In the external code, I was using Spotless to format the code. I'm keeping this (adding Spotless) but limiting it to Croissant classes for now. I'm very interested in our team programmatically formatting code in the backend, like we do in the frontend. I don't mind if we use a different command-line tool. Spotless could be considered a starting point.

Suggestions on how to test this:

Confirm the new feature flag is working.

Does this PR introduce a user interface change? If mockups are available, please link/include them here:

Yes, Croissant now appears out of the box:

Is there a release notes update needed for this change?:

Yes, included.

Additional documentation:

A good entry point for previewing doc changes: https://dataverse-guide--12130.org.readthedocs.build/en/12130/admin/discoverability.html#croissant-metadata-in-the-head-of-dataset-landing-pages

[pdurbin]

@landreev thanks for taking a look. I just changed the feature flag to a jvm option in a3d1247, as discussed in tech hours yesterday.

[github-actions]

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