Changes to Update a counterparty

added the new arc-testnet enum value to the counterparty/invite/onchainNetwork response property for the response status 200

added the new arc-testnet enum value to the counterparty/rails/items/oneOf[subschema #2]/disabledNetworks/items response property for the response status 200

added the new arc-testnet enum value to the counterparty/rails/items/oneOf[subschema #2]/network response property for the response status 200

added the new arc-testnet enum value to the counterparty/rails/items/oneOf[subschema #2]/preferredNetwork response property for the response status 200

endpoint added