signet: implement full block validation and generation

[Roasbeef]

A recently merged PR added support for signet to btcd, but in a way that affectively makes it a "light client" as it only verifies PoW, and not the new set of "virtual" signatures. In order to be able to create new btcd signed signets in the future, we should extend the logic during block validation (if signet is active) to verify the signatures. On the other end, when assembling a block via the GBT calls or the main CPU miner, we should add the new Coinbase OP_RETURN commitment as well.

[Yihen-Liu]

Does btcd add OP_RETURN commitment in Coinbase Tx for signet model? I don`t see this logic handling when I review the latest version of btcd.

[Roasbeef]

Support for "mining" a signet isn't implemented yet.