kernel: Add Block Header serialization method

Add `btck_block_header_to_bytes` serialization method to serialize a
`btck_BlockHeader` into an 80-byte buffer using `SpanWriter` to ensure
zero-allocation serialization.

src/kernel/bitcoinkernel.cpp

@@ -1390,6 +1390,16 @@ uint32_t btck_block_header_get_nonce(const btck_BlockHeader* header)
     return btck_BlockHeader::get(header).nNonce;
 }
 
+int btck_block_header_to_bytes(const btck_BlockHeader* header, unsigned char output[80])
+{
+    try {
+        SpanWriter{std::as_writable_bytes(std::span{output, 80})} << btck_BlockHeader::get(header);
+        return 0;
+    } catch (...) {
+        return -1;
+    }
+}
+
 void btck_block_header_destroy(btck_BlockHeader* header)
 {
     delete header;