Expose Expected Block Size to get_block_template RPC
With ``get_block_template`` RPC call we should able get expected block sizes like the ``expected_reward``.

Currently its internally calculated but its not exposed to RPC 

I believe its needed because it would be very useful blockchain explorers (for an example:  https://github.com/txstreet/processor/issues/2)

Related code: https://github.com/monero-project/monero/blob/master/src/cryptonote_core/blockchain.h