serve_server_wellknown should respect public_baseurl

This issue has been migrated from [#14355](https://github.com/matrix-org/synapse/issues/14355).

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**Description:**

If a server admin configures `serve_server_wellknown: true`, the response for `/.well-known/matrix/server` points other servers to `server_name:443` regardless of the value of `public_baseurl`. This breaks federation if Synapse is hosted on a dedicated subdomain and the admin reverse proxies `https://server_name/.well-known/matrix/` back to Synapse.

Start by tracing the serve_server_wellknown configuration and the handler for /.well-known/matrix/server. Compare the response URL with public_baseurl, then verify that the generated server address supports federation when Synapse is reverse-proxied from a dedicated subdomain.

Written by the indexing model from the issue text.