Add an Admin API to look up users by and list all threepids

[matrixbot]

This issue has been migrated from #13490.

---

Description:

• There should be a way to look up users by their threepid without needing to connect to the Synapse database directly. For example, a new optional parameter on https://matrix-org.github.io/synapse/latest/admin_api/user_admin_api.html?highlight=threepid#list-accounts.
  • Done in Dummy issue #14405
• It would also be useful if the List Accounts API could optionally include threepids when listing users to get an overview of all existing threepids on your server. Instead of needing to do SELECT * FROM user_threepids;