RPC and ZeroMQ APIs to support p2pool

Adds the following:

- "get_miner_data" to RPC API
- "json-miner-data" to ZeroMQ subscriber contexts

Both provide the necessary data to create a custom block template. They are used by p2pool.

Data provided:

- major fork version
- current height
- previous block id
- RandomX seed hash
- network difficulty
- median block weight
- coins mined by the network so far
- mineable mempool transactions

src/serialization/json_object.cpp

@@ -34,6 +34,7 @@
 #include <type_traits>
 
 #include "cryptonote_basic/cryptonote_basic_impl.h"
+#include "cryptonote_core/cryptonote_tx_utils.h"
 
 // drop macro from windows.h
 #ifdef GetObject
@@ -1411,6 +1412,27 @@ void fromJsonValue(const rapidjson::Value& val, cryptonote::rpc::output_distribu
   GET_FROM_JSON_OBJECT(val, dist.data.base, base);
 }
 
+void toJsonValue(rapidjson::Writer<epee::byte_stream>& dest, const cryptonote::tx_block_template_backlog_entry& entry)
+{
+  dest.StartObject();
+  INSERT_INTO_JSON_OBJECT(dest, id, entry.id);
+  INSERT_INTO_JSON_OBJECT(dest, weight, entry.weight);
+  INSERT_INTO_JSON_OBJECT(dest, fee, entry.fee);
+  dest.EndObject();
+}
+
+void fromJsonValue(const rapidjson::Value& val, cryptonote::tx_block_template_backlog_entry& entry)
+{
+  if (!val.IsObject())
+  {
+    throw WRONG_TYPE("json object");
+  }
+
+  GET_FROM_JSON_OBJECT(val, entry.id, id);
+  GET_FROM_JSON_OBJECT(val, entry.weight, weight);
+  GET_FROM_JSON_OBJECT(val, entry.fee, fee);
+}
+
 }  // namespace json
 
 }  // namespace cryptonote