Add support for extra transaction signature validation

- Restructures `STTx` signature checking code to be able to handle
  a `sigObject`, which may be the full transaction, or may be an object
  field containing a separate signature. Either way, the `sigObject` can
  be a single- or multi-sign signature.
- This is distinct from 550f90a75e (#5594), which changed the check in
  Transactor, which validates whether a given account is allowed to sign
  for the given transaction. This cryptographically checks the signature
  validity.

include/xrpl/protocol/STTx.h

@@ -88,7 +88,13 @@ public:
 
     // Outer transaction functions / signature functions.
     Blob
-    getSignature() const;
+    getSignature(STObject const& sigObject) const;
+
+    Blob
+    getSignature() const
+    {
+        return getSignature(*this);
+    }
 
     uint256
     getSigningHash() const;
@@ -119,13 +125,34 @@ public:
     getJson(JsonOptions options, bool binary) const;
 
     void
-    sign(PublicKey const& publicKey, SecretKey const& secretKey);
+    sign(
+        PublicKey const& publicKey,
+        SecretKey const& secretKey,
+        std::optional<std::reference_wrapper<SField const>> signatureTarget =
+            {});
 
-    /** Check the signature.
-        @return `true` if valid signature. If invalid, the error message string.
-    */
     enum class RequireFullyCanonicalSig : bool { no, yes };
 
+    /** Check the signature.
+        @param requireCanonicalSig If `true`, check that the signature is fully
+            canonical. If `false`, only check that the signature is valid.
+        @param rules The current ledger rules.
+        @param pSig Pointer to object that contains the signature fields, if not
+            using "this". Will most often be null
+        @return `true` if valid signature. If invalid, the error message string.
+    */
+    Expected<void, std::string>
+    checkSign(
+        RequireFullyCanonicalSig requireCanonicalSig,
+        Rules const& rules,
+        STObject const* pSig) const;
+
+    /** Check the signature.
+        @param requireCanonicalSig If `true`, check that the signature is fully
+            canonical. If `false`, only check that the signature is valid.
+        @param rules The current ledger rules.
+        @return `true` if valid signature. If invalid, the error message string.
+    */
     Expected<void, std::string>
     checkSign(RequireFullyCanonicalSig requireCanonicalSig, Rules const& rules)
         const;
@@ -155,12 +182,15 @@ public:
 
 private:
     Expected<void, std::string>
-    checkSingleSign(RequireFullyCanonicalSig requireCanonicalSig) const;
+    checkSingleSign(
+        RequireFullyCanonicalSig requireCanonicalSig,
+        STObject const* pSig) const;
 
     Expected<void, std::string>
     checkMultiSign(
         RequireFullyCanonicalSig requireCanonicalSig,
-        Rules const& rules) const;
+        Rules const& rules,
+        STObject const* pSig) const;
 
     Expected<void, std::string>
     checkBatchSingleSign(