[feat]: Add .well-known file for server discovery

[501st-alpha1]

Prerequisites

• I have searched for existing issues that already suggest this feature.

Describe the Feature Request

Per the Matrix Client-Server spec, a Matrix server can respond to requests on https://hostname/.well-known/matrix/client to help with auto-discovery. I checked that URL on my Synapse onion and it gave a 404, so it appears this hasn't been implemented.

Describe the Use Case

The reason I ask for this is despite the fact that the spec says:

If the returned status code is 404, then IGNORE.

I've run into a couple clients that instead throw errors and won't let me log in. I intend to report such cases as bugs, but if this was implemented that should work around such broken clients.

Describe Preferred Solution

No response

Describe Alternatives

No response

Anything else?

No response