feat(client): use non RFC 5780 STUN servers as backup

For context, see #40304 (closed), !449 (merged). In the client, we currently only try to use STUN servers that support NAT type discovery. But there aren't many of such servers, and censors are likely to target them, because they are not commonly used, especially because web WebRTC clients can't utilize this NAT type discovery feature.

However, NAT discovery is not strictly necessary for a Snowflake client to establish a connection with a proxy. In case the NAT type cannot be determined, the client would simply try to connect to unrestricted proxies, and with !392 (merged) it will even try to connect to a restricted one first in such a case.

But, if all STUN servers are blocked for a client, they won't be able to establish a connection at all (unless they have a public IP, which is unlikely).

So I propose to have two separate lists of STUN servers for clients: ones that support NAT discovery, and the backup ones that do not necessarily support it. Servers with more "collateral freedom", such as the Google's STUN server, should be on that list.

This issue is sort of a continuation of #40376 (closed).