Merge bitcoin#25019: parse external signer master fp as bytes in ExternalSigner::SignTransaction

2a22f03 parsing external signer master fingerprint string as bytes instead of caring for lower/upper case in ExternalSigner::SignTransaction (avirgovi)

Pull request description:

  Some external signers scripts may provide master fingerprint in uppercase format. In that case core will fail with `Signer fingerprint 00000000 does not match any of the inputs` as it only works with lowercase format. Even if the fingerprints match, yet one is lowercase the other uppercase.

  ExternalSigner::SignTransaction is the only place where it is needed IMO, as changing it in other places may break the communication with the external signer (i.e. enumerating with lowercase may not find the device).

ACKs for top commit:
  achow101:
    ACK 2a22f03
  theStack:
    Code-review ACK 2a22f03
  Sjors:
    utACK 2a22f03

Tree-SHA512: f3d84b83fb0b5e06c405eaf9bf20a2fa864bf4172fd4de113b80b9b9a525a76f2f8cf63031b480358b3a7666023a2aef131fb89ff50448c66df3ed541da10f99

Author: achow101

src/external_signer.cpp

@@ -74,11 +74,12 @@ bool ExternalSigner::SignTransaction(PartiallySignedTransaction& psbtx, std::str
     // Serialize the PSBT
     CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
     ssTx << psbtx;
-
+    // parse ExternalSigner master fingerprint
+    std::vector<unsigned char> parsed_m_fingerprint = ParseHex(m_fingerprint);
     // Check if signer fingerprint matches any input master key fingerprint
     auto matches_signer_fingerprint = [&](const PSBTInput& input) {
         for (const auto& entry : input.hd_keypaths) {
-            if (m_fingerprint == strprintf("%08x", ReadBE32(entry.second.fingerprint))) return true;
+            if (parsed_m_fingerprint == MakeUCharSpan(entry.second.fingerprint)) return true;
         }
         return false;
     };