kernel: Add function for copying block data to C header

This adds a function for streaming bytes into a user-owned data
structure.

Use it in the tests for verifying the implementation of the validation
interface's `BlockChecked` method.

src/kernel/bitcoinkernel.cpp

@@ -967,6 +967,17 @@ const btck_Transaction* btck_block_get_transaction_at(const btck_Block* block, s
     return btck_Transaction::ref(&btck_Block::get(block)->vtx[index]);
 }
 
+int btck_block_to_bytes(const btck_Block* block, btck_WriteBytes writer, void* user_data)
+{
+    try {
+        WriterStream ws{writer, user_data};
+        ws << TX_WITH_WITNESS(*btck_Block::get(block));
+        return 0;
+    } catch (...) {
+        return -1;
+    }
+}
+
 void btck_block_destroy(btck_Block* block)
 {
     delete block;

src/kernel/bitcoinkernel.h

@@ -249,6 +249,11 @@ typedef void (*btck_ValidationInterfacePoWValidBlock)(void* user_data, btck_Bloc
 typedef void (*btck_ValidationInterfaceBlockConnected)(void* user_data, btck_Block* block, const btck_BlockTreeEntry* entry);
 typedef void (*btck_ValidationInterfaceBlockDisconnected)(void* user_data, btck_Block* block, const btck_BlockTreeEntry* entry);
 
+/**
+ * Function signature for serializing data.
+ */
+typedef int (*btck_WriteBytes)(const void* bytes, size_t size, void* userdata);
+
 /**
  * Whether a validated data structure is valid, invalid, or an error was
  * encountered during processing.
@@ -389,11 +394,6 @@ typedef uint8_t btck_ChainType;
 #define btck_ChainType_SIGNET ((btck_ChainType)(3))
 #define btck_ChainType_REGTEST ((btck_ChainType)(4))
 
-/**
- * Function signature for serializing data.
- */
-typedef int (*btck_WriteBytes)(const void* bytes, size_t size, void* userdata);
-
 /** @name Transaction
  * Functions for working with transactions.
  */
@@ -983,6 +983,21 @@ BITCOINKERNEL_API size_t BITCOINKERNEL_WARN_UNUSED_RESULT btck_block_count_trans
 BITCOINKERNEL_API const btck_Transaction* BITCOINKERNEL_WARN_UNUSED_RESULT btck_block_get_transaction_at(
     const btck_Block* block, size_t transaction_index) BITCOINKERNEL_ARG_NONNULL(1);
 
+/**
+ * @brief Serializes the block through the passed in callback to bytes.
+ * This is consensus serialization that is also used for the P2P network.
+ *
+ * @param[in] block     Non-null.
+ * @param[in] writer    Non-null, callback to a write bytes function.
+ * @param[in] user_data Holds a user-defined opaque structure that will be
+ *                      passed back through the writer callback.
+ * @return              0 on success.
+ */
+BITCOINKERNEL_API int btck_block_to_bytes(
+    const btck_Block* block,
+    btck_WriteBytes writer,
+    void* user_data) BITCOINKERNEL_ARG_NONNULL(1, 2);
+
 /**
  * Destroy the block.
  */

src/kernel/bitcoinkernel_wrapper.h

@@ -548,6 +548,11 @@ public:
     }
 
     MAKE_RANGE_METHOD(Transactions, Block, &Block::CountTransactions, &Block::GetTransaction, *this)
+
+    std::vector<std::byte> ToBytes() const
+    {
+        return write_bytes(get(), btck_block_to_bytes);
+    }
 };
 
 inline void logging_disable()