Merge bitcoin/bitcoin#24836: add RPC (-regtest only) for testing package policy

e866f0d066 [functional test] submitrawpackage RPC (glozow)
fa076515b0 [rpc] add new submitpackage RPC (glozow)

Pull request description:

  It would be nice for LN/wallet/app devs to test out package policy, package RBF, etc., but the only interface to do so right now is through unit tests. This PR adds a `-regtest` only RPC interface so people can test by submitting raw transaction data. It is regtest-only, as it would be unsafe/confusing to create an actual mainnet interface while package relay doesn't exist.

  Note that the functional tests are there to ensure the RPC interface is working properly; they aren't for testing policy itself. See src/test/txpackage_tests.cpp.

ACKs for top commit:
  t-bast:
    Tested ACK against eclair e866f0d066
  ariard:
    Code Review ACK e866f0d0
  instagibbs:
    code review ACK e866f0d066

Tree-SHA512: 824a26b10d2240e0fd85e5dd25bf499ee3dd9ba8ef4f522533998fcf767ddded9f001f7a005fe3ab07ec95e696448484e26599803e6034ed2733125c8c376c84

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);

src/util/error.h

@@ -32,6 +32,7 @@ enum class TransactionError {
     MAX_FEE_EXCEEDED,
     EXTERNAL_SIGNER_NOT_FOUND,
     EXTERNAL_SIGNER_FAILED,
+    INVALID_PACKAGE,
 };
 
 bilingual_str TransactionErrorString(const TransactionError error);