DNSAdmin: Allow plugins to have optional/fixed node roles

As a System Administrator, Developer and user of a third party DNS service,

I would like the ability for custom dnsadmin plugins to be able to specify which "node roles" are supported by that plugin (as per https://api.docs.cpanel.net/guides/guide-to-custom-dnsadmin-plugins/guide-to-custom-dnsadmin-plugins-node-configuration-files/#node-roles ).

As far as I can tell, only cPanel/WHM to/from cPanel/WHM supports all 3 node roles, the "in-built" Softlayer and VPSNet dnsadmin (and, the custom one I am developing) only actually support "Write-only" mode (in that changes made in cPanel/WHM will go to the remote system - such as Softlayer - but changes as Softlayer etc do not then feed back to cPanel/WHM.

Allowing developers to hardcode "dnsrole" in the setup:get_config() will help avoid user confusion as to why an option does not "appear" to work and simplify setup in some circumstances.

It would also be nice to add additional text to the setup page (such as explanatory notes).