[rpc] add new submitpackage RPC

It could be unsafe/confusing to create an actual mainnet interface while
package relay doesn't exist. However, a regtest-only interface allows
wallet/application devs to test current package policies.

src/util/error.cpp

@@ -35,6 +35,8 @@ bilingual_str TransactionErrorString(const TransactionError err)
             return Untranslated("External signer not found");
         case TransactionError::EXTERNAL_SIGNER_FAILED:
             return Untranslated("External signer failed to sign");
+        case TransactionError::INVALID_PACKAGE:
+            return Untranslated("Transaction rejected due to invalid package");
         // no default case, so the compiler can warn about missing cases
     }
     assert(false);