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

This issue has been migrated from [#13490](https://github.com/matrix-org/synapse/issues/13490).

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**Description:**

- [x] 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 #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;`

First review the completed lookup portion in #14405, then read the List Accounts API documentation at user_admin_api.html. Inspect how the List Accounts API currently exposes users and compare it with the user_threepids table mentioned in the issue. Done means callers can optionally include all users’ threepids when listing accounts.

Written by the indexing model from the issue text.