Require producer authority ("BP key") to be a single K1 key for each producer

[spoonincode]

The original EOSIO protocol allowed a producer to register its producer key with a key on either the K1 or R1 curve. With the introduction of WTMSIG_BLOCK_SIGNATURES protocol feature, the producer is able to register a producer authority with multiple keys and weights (much like an authority on an account's permission). With the WEBAUTHN_KEY protocol feature, producer authorities can technically include a PUB_WA in their authority; but blocks signed with SIG_WA have been dropped from the P2P layer for some time.

It appears in practice BPs do not make use of all this flexibility: they use a single key on the K1 curve.

Changes to the protocol are being explored for faster finality. Of particular note is that a new protocol being designed makes use of Schnorr signatures. These Schnorr signatures, plus a desire to simply reduce complexity in the protocol, leads us to propose that once this new faster finality protocol feature is enabled, a producer may only have a single key on the K1 curve registered regardless whether WTMSIG_BLOCK_SIGNATURES has been activated or not.

The exact mechanics of how (or if) the protocol feature can be activated with active producer authorities that violate this rule is outside the scope of this issue. We are interested in hearing any feedback against the removal of multi-key producer authorities and usage of R1 keys.

[spoonincode]

The prompt for this was early IF work; the implementation ended up being based on BLS instead.