wallet, refactor: Convert uint256 to Txid in wallet interfaces

In most cases throughout the wallet, the implicit conversion from `Txid` to
`const uint256&` works. However, `commitBumpTransaction` requires a `uint256&`
out parameter, so `bumped_txid` in `feebumper::CommitTransaction` is also
updated here to use `Txid`.

src/wallet/rpc/spend.cpp

@@ -1166,7 +1166,7 @@ static RPCHelpMan bumpfee_helper(std::string method_name)
             throw JSONRPCError(RPC_WALLET_ERROR, "Can't sign transaction.");
         }
 
-        uint256 txid;
+        Txid txid;
         if (feebumper::CommitTransaction(*pwallet, hash, std::move(mtx), errors, txid) != feebumper::Result::OK) {
             throw JSONRPCError(RPC_WALLET_ERROR, errors[0].original);
         }