fix(terraform): Handle error when custom domain is not enabled

[thomas-auth0]

🔧 Changes

Fix for #1102
Skip the generation of custom domains resources if the feature is not activated yet on the tenant:

$ auth0 tf generate --output-dir tmp-tfgen
Fetching data from Auth0... failed

=== <redacted>.us.auth0.com error

 ▸    403 Forbidden: There must be a verified credit card on file to perform this operation.

📚 References

https://auth0.com/docs/api/management/v2/custom-domains/post-verify
#854

🔬 Testing

Added unit test and tested manually.

📝 Checklist

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)