Fix: missing LogCredentialType causes exception while reading log data

[explody]

While fetching and reading logs, we began hitting an unhandled exception of

ValueError: 'CERTIFICATE' is not a valid LogCredentialType

We determined this was from recently deploying FastPass and device integrations, which caused a new authentication type to appear in the logs.

I tracked this down to okta/models/log_credential_type.py, where based on the existing lines, the "CERTIFICATE" type was simply missing, so I've added that.