UI for contacting bridged users/rooms by third party ID.

### Your use case

#### What would you like to do?

As a user, I'd like to be able to contact users or rooms on protocols which are bridged to Matrix by providing their thirdparty IDs.

This has existed for locally bridged users ~10 years via the thirdparty lookup APIs on the CS API: https://spec.matrix.org/v1.19/client-server-api/#third-party-networks - but we've never coherently exposed it in Element. Meanwhile MSC4517 adds support for remote bridged users.

#### Why would you like to do it?

If I want to DM someone on XMPP - e.g. xmpp:matthew@xmpp.org.uk right now, I have to know to go read https://github.com/matrix-org/matrix-bifrost/wiki/Address-syntax and manually convert it to be `@_xmpp_matthew=xmpp.org.uk:matrix.org` and enter it into the app.

Similarly, if I want to join a MUC on XMPP, I have to know to manually convert `@xsf:xmpp.org` to `#_xmpp_xsf_xmpp.org:matrix.org`.

This is unbelievably user hostile.

#### How would you like to achieve it?

Reuse the existing UI that room directories provide for querying third party room directories to also let you query third party servers and protocols when contacting a given user ID or room ID.

It would look something like this:

Start by reviewing the existing room-directory UI for third-party room searches and the Client-Server API third-party network lookup section, then read MSC4517 for remote bridged users. The finished work should let users enter third-party IDs when contacting a user or room, query the relevant bridged protocols, and use the results without manually converting Matrix IDs.

Written by the indexing model from the issue text.