Calls: use all relays' TURN servers, not just one

Our ice_servers() function only returns the TURN server of one of the relays

But maybe we should return all of the relay’s TURN servers (except for “hidden” ones’ I guess Relay is not removed from Connections after deletion (Android 2.56) - #7 by r10s), and make it up to the WebRTC agent to choose the best? Especially if one of the relay’s TURN server is returned by the relay but is not working (this came up in).

Is there anything against this?

For reference, there was a similar MR for quota: feat: connectivity view: quota for all transports by Simon-Laux · Pull Request #7630 · chatmail/core · GitHub.
Is turning the metadata field into a BTreeMap again the way to go?

metadata has been turned into a BTreeMap in use relay's maxsmtprecipients metadata for smtp chunking, to remove another is_chatmail use by hpk42 · Pull Request #8426 · chatmail/core · GitHub.
Now there is this MR: