Merge bitcoin/bitcoin#25748: refactor: Avoid copies in FlatSigningProvider Merge

fa3f15f2dd refactor: Avoid copies in FlatSigningProvider Merge (MacroFake)

Pull request description:

  `Merge` will create several copies unconditionally:
  * To initialize the args `a`, and `b`
  * `ret`, which is the merge of the two args

  So change the code to let the caller decide how many copies they need/want:
  * `a`, and `b` must be explicitly moved or copied by the caller
  * `ret` is no longer needed, as `a` can be used for it in place "for free"

ACKs for top commit:
  achow101:
    ACK fa3f15f2dd
  furszy:
    looks good, ACK fa3f15f2
  ryanofsky:
    Code review ACK fa3f15f2dd. Confirmed that all the places `std::move` was added the argument actually did seem safe to move from. Compiler enforces that temporary copies are explicitly created in non-move cases.

Tree-SHA512: 7c027ccdea1549cd9f37403344ecbb76e008adf545f6ce52996bf95e89eb7dc89af6cb31435a9289d6f2eea1c416961b2fb96348bc8a211d550728f1d99ac49c

src/wallet/rpc/spend.cpp

@@ -644,7 +644,7 @@ void FundTransaction(CWallet& wallet, CMutableTransaction& tx, CAmount& fee_out,
                     throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Unable to parse descriptor '%s': %s", desc_str, error));
                 }
                 desc->Expand(0, desc_out, scripts_temp, desc_out);
-                coinControl.m_external_provider = Merge(coinControl.m_external_provider, desc_out);
+                coinControl.m_external_provider.Merge(std::move(desc_out));
             }
         }
     }