feat: fix papercuts around domain verifications

[ohookins]

There are two problems with the okta_domain_verification and okta_email_domain_verification resources right now:

1. If the custom domain / email domain is already verified by DNS, attempting to create these resources (by triggering another verification) will fail with a 400 error.
2. There's no way to import the current verification status of the domain.

I've attempted to solve both of these problems by

1. Querying the domain verification status before attempting to verify again, so avoiding triggering an API request which unfortunately is not idempotent.
2. Allowing the user to import the domain verification and avoid case (1) altogether if desired.

Fixes #1909