Merge bitcoin/bitcoin#28417: contrib/signet/miner updates

fb6d51e signet/miner: Use argparse exclusive groups (Anthony Towns)
338a266 signet/miner: add support for a poolnum/poolid tag in mined blocks (Anthony Towns)
409ab7d signet/miner: add Generate.mine function (Anthony Towns)
7b31332 signet/miner: add Generate.gbt function (Anthony Towns)
85c5c0b signet/miner: add Generate.next_block_time function (Anthony Towns)
5540e6c signet/miner: move next_block_* functions into new Generator class (Anthony Towns)
35f4631 signet/miner: rename do_decode_psbt to decode_psbt (Anthony Towns)
aac040b signet/miner: drop create_coinbase function (Anthony Towns)
16951f5 signet/miner: drop do_createpsbt function (Anthony Towns)
3aed0a4 signet/miner: drop get_reward_address function (Anthony Towns)

Pull request description:

  Refactors the code a bunch, and adds `--poolnum` / `--poolid` options so that signers can tag their coinbases in a way that explorers can recognise (see also bitcoin-data/mining-pools#82 and mempool/mempool#2903).

  The refactoring in particular helps enable the "try using inquisition's getblocktemplate, and if that doesn't work fall back to core's getblocktemplate" logic, as described/implemented in bitcoin-inquisition/bitcoin#7

ACKs for top commit:
  achow101:
    ACK fb6d51e
  danielabrozzoni:
    Code review ACK fb6d51e

Tree-SHA512: d84095c4045ab196685b847e04ce2cdaedf387bc2527430ede918318dc5b70bf3d87b754264016f895f506fac70d4fdea5ef3cd8c3c375fd586afeae01e045e5