Merge bitcoin#30253: refactor: performance-for-range-copy in psbt.h

fab01b5 refactor: performance-for-range-copy in psbt.h (MarcoFalke)

Pull request description:

  A copy of the partial signatures is not required before serializing them.

  For reference, this was found by switching the codebase to `std::span` (not sure why it wasn't found with `Span` though):

  ```
  ./psbt.h:240:23: error: loop variable is copied but only used as const reference; consider making it a const reference [performance-for-range-copy,-warnings-as-errors]
    240 |             for (auto sig_pair : partial_sigs) {
        |                       ^
        |                  const  &

ACKs for top commit:
  tdb3:
    ACK fab01b5
  theStack:
    utACK fab01b5

Tree-SHA512: b55513d8191118499716684190ee568d171b50ae9171d246ca6e047f0cfd3ec14c9453d721e88af55e47bb41fa66cbafdbfb47bc5f9b8d82789e0a9b634b371b

Author: fanquake

src/psbt.h

@@ -237,7 +237,7 @@ struct PSBTInput
 
         if (final_script_sig.empty() && final_script_witness.IsNull()) {
             // Write any partial signatures
-            for (auto sig_pair : partial_sigs) {
+            for (const auto& sig_pair : partial_sigs) {
                 SerializeToVector(s, CompactSizeWriter(PSBT_IN_PARTIAL_SIG), Span{sig_pair.second.first});
                 s << sig_pair.second.second;
             }