fix: Digital Certificate creation

[AndreZiviani]

API return a comma separated list of subject names, we need to split it to a list, maybe doing on the SDK would be better. Also order shouldn't matter so use Set instead

Fixes:

╷
│ Error: Provider produced inconsistent result after apply
│ 
│ When applying changes to azion_digital_certificate.test, provider "provider[\"registry.terraform.io/aziontech/azion\"]" produced
│ an unexpected new value: .certificate_result: inconsistent values for sensitive attribute.
│ 
│ This is a bug in the provider, which should be reported in the provider's own issue tracker.
╵

[AndreZiviani]

I have read the CLA Document and I hereby sign the CLA

[maxwelbm]

@AndreZiviani Error occurs due to incorrect use of the structure that does not support the subject_name field.

produced an unexpected new value: .certificate_result:
│ inconsistent values for sensitive attribute.

https://registry.terraform.io/providers/aziontech/azion/latest/docs/resources/digital_certificate