Merge bitcoin/bitcoin#29032: signet: omit commitment for some trivial challenges

6ee32aaaca test: signet tool genpsbt and solvepsbt commands (Sjors Provoost)
0a99d99fe4 signet: miner skips PSBT step for OP_TRUE (Sjors Provoost)
cdfb70e5a6 signet: split decode_psbt miner helper (Sjors Provoost)

Pull request description:

  [BIP325](https://github.com/bitcoin/bips/blob/master/bip-0325.mediawiki) mentions the following rule:

  > In the special case where an empty solution is valid (ie scriptSig and scriptWitness are both empty) this additional commitment can optionally be left out. This special case is to allow non-signet-aware block generation code to be used to test a custom signet chain where the challenge is trivially true.

  Such a signet can be created using e.g. `-signetchallenge=51` (`OP_TRUE`). However `contrib/signet/miner` won't omit the commitment.

  This PR improves the miner by skipping the PSBT for known trivial scripts (just `OP_TRUE` and trivial pushes for now). This prevents it from appending the 4 byte signet header to the witness commitment, as allowed by the above rule.

  ---

  Previously the script would fail with `PSBT signing failed`, making it difficult to mine. This is no longer the case.

ACKs for top commit:
  achow101:
    ACK 6ee32aaaca
  theStack:
    re-ACK 6ee32aaaca
  danielabrozzoni:
    ACK 6ee32aaaca

Tree-SHA512: e47fbf471f2909286a6c1c073799ea388b9c19551afcce96cf9af45cc48d25c02f1e48e08861a88b604361e2c107a759d5baf393da8a37360de419f31651758a

test/functional/test_framework/blocktools.py

@@ -81,6 +81,9 @@ DIFF_4_N_BITS = 0x1c3fffc0
 DIFF_4_TARGET = int(DIFF_1_TARGET / 4)
 assert_equal(uint256_from_compact(DIFF_4_N_BITS), DIFF_4_TARGET)
 
+# From BIP325
+SIGNET_HEADER = b"\xec\xc7\xda\xa2"
+
 def nbits_str(nbits):
     return f"{nbits:08x}"