Allow terraform to add credentials to models

[dparv]

Requirements

When adding a model that needs to use a custom credential, there should be an option to specify a credential, e.g.:

juju add-model tetsing --credential azure-cloud

### Urgency

Casually wishlisting

### Notes & References

_No response_

[dparv]

Updated this to Casually wishlisting, as having a single credential for a controller works just fine when adding models to Azure for exmaple.

[hmlanigan]

This is a small feature request as it requires a schema change to add credential to the model resource.