kernel: Split ParseSighashString

This split is done in preparation for the next commit where the
dependency on UniValue in the kernel library is removed.

doc/release-notes-28113.md

@@ -0,0 +1,7 @@
+RPC Wallet
+----------
+
+- The `signrawtransactionwithkey`, `signrawtransactionwithwallet`,
+  `walletprocesspsbt` and `descriptorprocesspsbt` calls now return more
+  specific RPC_INVALID_PARAMETER instead of RPC_PARSE_ERROR if their
+  sighashtype argument is malformed or not a string.