Add support for `cleos convert` families

[maoueh]

Request on Telegram.

$ cleos convert
ERROR: RequiredError: Subcommand required
Pack and unpack transactions
Usage: cleos convert SUBCOMMAND

Subcommands:
  pack_transaction            From plain signed json to packed form
  unpack_transaction          From packed to plain signed json form
  pack_action_data            From json action data to packed form
  unpack_action_data          From packed to json action data form