Publicize: add Fediverse meta tag on posts when available (#38198)

* Publicize: add Fediverse meta tag on posts when available

See https://blog.joinmastodon.org/2024/07/highlighting-journalism-on-mastodon/

If we have an existing Mastodon connection on the site, let's add it as meta tag on each post.

* Improve logic to support global connections.

1. We now keep all existing Mastodon connections for a post, as long as that post was published to that connection. If you've opted not to share that post to Mastodon, we do not keep that connection.
2. We keep both your own connections as well as any other global connections on the site.
3. We then sort through those connections:
    * We prioritize your own connections, and pick the first of them,
    * If you, the author, haven't published that specific post to a Mastodon connection, we look for global Mastodon connections. If there are some, we pick the first.

See discussion here:
Automattic/jetpack#38198 (comment)

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/9935380055

Upstream-Ref: Automattic/jetpack@fe17eb1

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