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/daemon/daemon.cpp

@@ -120,6 +120,7 @@ public:
       if (shared)
       {
         core.get().get_blockchain_storage().add_block_notify(cryptonote::listener::zmq_pub::chain_main{shared});
+        core.get().get_blockchain_storage().add_miner_notify(cryptonote::listener::zmq_pub::miner_data{shared});
         core.get().set_txpool_listener(cryptonote::listener::zmq_pub::txpool_add{shared});
       }
     }