fix: send SDP answer if ICE gathering is taking long to complete

See snowflake-webext!55 (merged). Probably need to implement it here as well (snowflake-webext!55 (comment 2851693)).

#40164 has the same title, but suggests a different approach, maybe close then this in favor of that.

Also I think it's best if we can still send the end-of-candidates marker in the SDP so that the remote peer knows not to expect any more ICE candidates.