Allow bridges to set MyFamily

Currently, families are not supported for bridge relays at all. Bridge operators are warned against listing bridge fingerprints in the MyFamily of their other relays. But the code also ensures that, even if MyFamily for a bridge is specified, it is not used. This was set way back in #4657 (closed).

The reasoning for not providing MyFamily support for bridges was that making the fingerprint of a bridge public allows a censor to download the bridge descriptor which contains the IP address and bridge lines. However, this reasoning is outdated as some pluggable transports like Meek and Snowflake do not rely on the secrecy of their location or configurations to resist blocking.

These transports also tend to be run as very large bridges, with a high volume of traffic and a high number of users. That puts them in a similar category to exits in terms of what we need from operators and it happens that we currently have more than operator running both a Meek/Snowflake bridge and an exit (though I think one of these operators has both in the same /16).

My feeling is that allowing bridges to set MyFamily outweighs the risk/detriment of a small handful of bridge operators running obfs4 or webtunnel bridges disclosing their bridge fingerprints. It makes sense to keep the warnings and guidance on not setting it for bridge operators (perhaps with some updated wording), but we should allow operators that are running both unhidden bridges and exits to protect users from creating circuits that would allow end to end timing attacks. I'll also note that the risk is in operators adding their bridge fingerprints to the torrc files of other relays. So there's not really any harm in respecting it once it's added. The warnings are really all that are protecting against that anyway.