Refactor AuditLogFinder public interface

What does this MR do?

Split ID finding out of common execute interface.

AuditLogFinder now relies on methods provided by FinderMethods module to perform ID lookup. This ensures a consistent return type for execute (i.e always return AuditEvent::ActiveRecord_Relation) instead of AuditEvent::ActiveRecord_Relation and AuditEvent (if id is provided).

This MR also ensure both Group Audit Events and Audit Events API use the newly exposed find_by! method in AuditLogFinder.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team