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_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()