Replace the `rtc_foci` well-known support with the dedicated CS API.

[pixlwave]

Reading the rtc_foci from the well-known is no longer part of MSC4143, it was replaced by a new CS API. The switch should look something like this:

• Add support to the SDK (and Ruma?) for calling the /_matrix/client/v1/rtc/transports endpoint.
• Add support for caching the response (either automatically similarly to /versions or manually like we do for /capabilities I believe).
• TBC: Remove the well-known caching as it was added specifically for this purpose and we're moving away from reading configuration via the .well-known file.