Add ability to manage custom domains

[emerkle826]

It seesm the DevOps API has the ability to add/list custom domains for use with private links in Astra. See here:
https://docs.datastax.com/en/astra/astra-api-docs/_attachments/devops-api/index.html#tag/Private-Link/operation/setCustomDomains

NOTE: There doesn't appear to be a way to remove custom domains at the moment.