Add `/.well-known/matrix/client` for Matrix clients

Add /.well-known/matrix/client for Matrix clients (well-known)

Previous MR where we added /.well-known/matrix/server: !2031 (merged)

Part of https://github.com/matrix-org/matrix-ansible-private/issues/6361 and the Gitter sunset: https://github.com/vector-im/roadmap/issues/26

Testing strategy

1. Add to "matrix": { "wellKnownServer": "gitter.ems.host:443" }, to config/config.user-overrides.json
2. Visit http://localhost:5000/.well-known/matrix/server
3. Add to "matrix": { "wellKnownClient": { "homeserverBaseUrl": "https://gitter.ems.host" } }, to config/config.user-overrides.json
4. Visit http://localhost:5000/.well-known/matrix/client

Todo

• Make sure /.well-known/matrix/client also includes CORS headers necessary so another webapp can fetch it

  const res = await fetch('http://localhost:5000/.well-known/matrix/client');
  const data = await res.json();
  console.log('data', data);