Expose tombstone status in room details

[RamReso]

Exposes tombstoned and replacement_room in room details on admin API endpoint GET /_synapse/admin/v1/rooms/<room_id>. Resolves #18347

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
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• Code style is correct (run the linters)

[reivilibre]

Seems good to me otherwise though, thank you!

[reivilibre]

please add short descriptions of these to the bullet points above, bear in mind not everyone will be familiar with these terms.
Should also note that a room can be tombstoned without having a replacement room, as that is probably not obvious

[reivilibre]

(please hit the 'Re-request review' button at the top right of the page, next to my name after the changes, to make sure I see this. Thank you)