Add a cop that dissallowd the use of `execute.find_by` on a Finder

Instead we should use Finder#find_by defined in FinderMethods, only available after !3709 (closed) is merged.

The following discussion from !3709 (closed) should be addressed:

@reprazent started a discussion: (+2 comments)

Add a cop that dissallows finders using the finder.execute.find(_by) pattern. Instead we should use finder.find as we have in IssuableFinder#find_by