Add config option to specify admin contact or support page (MSC1929)

If an admin contact or support page is set in the configuration,

there is already a `admin_contact`, it might be usefull to add extra config that then uses ([MSC1929](https://github.com/matrix-org/matrix-spec-proposals/pull/1929)) to announce who is in charge of an homeserver.

**synapse should then serve this well-known url!**

refs:
- https://github.com/matrix-org/matrix-federation-tester/issues/157
- https://github.com/the-draupnir-project/Draupnir/pull/150
- https://spec.matrix.org/v1.10/client-server-api/#getwell-knownmatrixsupport
- https://github.com/ruma/ruma/pull/1751

help to fight spam and report spam accounts to the homeserver admins etc ...