Introduce MergeTrain::BaseService

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The following discussion from !51433 (merged) should be addressed:

• @allison.browne started a discussion: (+1 comment)

  If we don't pass or use project, user or params I don't think it still makes sense to inherit from BaseService. Instead we could create a new MergeTrain::BaseService class or include BaseServiceUtility directly in MergeTrains::RefreshService so that this perform method looks like:

  def perform(target_project_id, target_branch)
   ::MergeTrains::RefreshService
          .new
          .execute(target_project_id, target_branch)
  end

  See the note in the base service:

  # TODO: New services should consider inheriting from
  #       BaseContainerService, or create new base class:
  #       https://gitlab.com/gitlab-org/gitlab/-/issues/216672

  or alternately if we do use BaseService we can pass target_project on initialization:

   ::MergeTrains::RefreshService
          .new(target_project)
          .execute(target_branch)

  My preference would be to prefer composition over inheritance and use include BaseServiceUtility directly in MergeTrain::RefreshService.