feat: allow integrations to refresh content layer data

[ascorbic]

Changes

Adds a new refreshContent helper to the 'astro:server:setup' hook context. This allows an integration to trigger a refresh of the content layer. Uses for this could be to enable a webhook endpoint during dev, or to connect to a CMS and watch for updates.

It can optionally make this a selective sync by passing a loaders array, which means it will only sync collections that use the loader with that name. It can also pass an optional context object, which is made available to the loader. This can be used to do things such as pass an incoming webhook body.

Includes some cherry-picked commits from #11902

Testing

Added an example webhook endpoint to the content layer fixture, and a test that uses this.

Docs

This will need an update to the integration API docs. It includes a changeset with usage examples.

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[ematipico]

@ascorbic can you link to us the chapter of the RFC that talks about about this new feature? If there isn't, we should add it before merging the PR

[ascorbic]

@ematipico I proposed it here, though haven't added the final API to the PR yet

[ematipico]

It should be added to the document, yes

[ascorbic]

On it. Also need to work out why the test is failing here but not locally!

[ascorbic]

RFC updated with a new section

[sarah11918]

Really nice addition here @ascorbic ! Just had some suggestions for your consideration to maybe tighten up with more direct instructions, and I always prefer to front load the name of the new thing being added so that if people stop reading after a few words, they know which property to look up or search for! 😄