Add PKNIC implementation

[CodelineNL]

The following operations have been implemented for PKNIC:

• register() - PKNIC doesn't support Admin contact creation, yet it's required by the provision-provider-domain-names; thus, the parameter is required but ignored;
• renew() - PKNIC always renews domains for 2 years, so the input is ignored;
• getInfo()
• updateNameservers()
• getEPPCode() - at least for sandbox accounts, PKNIC always returns sth like "contact staff@pknic.net.pk";

The following operations aren't supported by PKNIC:

• poll()
• domainAvailabilityCheck()
• transfer()
• initiateTransfer()
• finishTransfer()
• updateRegistrantContact() - PKNIC supports updates only for billing and technical contacts via API
• updateIpsTag()
• setLock()
• setAutoRenew()

NB. For production instance, API URL should be set up via configuration;