Taint untrusted link authentication keys

We should taint untrusted link auth keys, and then downgrade connections using tainted keys to protocol warnings.

Link auth keys from the following sources are trusted:

• hard-coded authorities
• the consensus signed by hard-coded authorities

Link auth keys from the following sources are untrusted:

• hardcoded fallback dirs, because relay keys change over time
• our state file (if not confirmed in the consensus), because relay keys change over time
• onion service descriptors, because they come from untrusted services
• onion service introduce cells, because they come from untrusted clients

Split off #26924 (moved).