[AP] Add support for Replies collections

Mastodon did this and I believe these are the relevant pull requests.

The goal is for Pleroma to be able to publish and receive a list of valid replies to a post to improve thread reconstruction.

• https://github.com/tootsuite/mastodon/pull/10106
• https://github.com/tootsuite/mastodon/pull/10128
• https://github.com/tootsuite/mastodon/pull/11582

cc @rinpatch for further feedback