add getter to BlockTag to access the block number

[marcinczenko]

Adds a getter to access the block number for BlockTag with number. Returns Option[BlockTag]:

let blockTag = BlockTag.init(42.u256)
if blockNumber =? blockTag.number:
  # use block number
else:
  # it was not a BlockTag with number
  # but one of the predefined tags 
  # (earliest, latest, or pending)

[AuHau]

LGTM, thanks! 👍