DRY up `ee/app/finders/security/training_providers/` classes

Follow up from here !111592 (comment 1344652999)

Implementation plan:

• Went to https://gitlab.com/gitlab-org/gitlab/-/tree/master/ee/app/finders/security/training_providers and try to move the common methods used in sub classes to base class
• This is came as a follow up from here !111592 (comment 1344652999) so we need to wait until the new provider is added
• Adjust the specs as needed