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/cryptonote_basic/fwd.h

@@ -33,4 +33,5 @@ namespace cryptonote
   struct block;
   class transaction;
   struct txpool_event;
+  struct tx_block_template_backlog_entry;
 }