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

[Twi1ightSparkle]

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 Add an Admin API endpoint for looking up users based on 3PID #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;