add activate_network_merchants and deactivate_network_merchants api

# Activates merchants for the given catalog by network_merchant_id.
  #
  # Note from PopShops
  # network merchant ids is a string
  # this actually has to be a combination value of the Rakuten PopShops' network id and the network merchant id.
  # These are combined using a    dash '-' character like: {network_id}-{network_merchant_id}.

  # For example, if you wanted to add 'Things From Another World' (network_merchant_id=8908) from the ShareASale network (network_id = 1).
  # By adding the two values together you get the following network_merchant_id: 1-8908.

  # You would pass in the following: network_merchant_id=1-8908
  # For example: network_merchant_ids = '1-8908, 2-3233'