Investigate: Handle setting sharding key

1. Our current factory for ee/app/models/merge_requests/approval_rule.rb class fails validation so probably we need to address that first (needs a sharding id)
2. I see we have a validation to to make sure validate :ensure_single_sharding_key. We don’t currently have any logic to handle setting the id on creation like how the existing ApprovalRulesMergeRequest does via:

    before_create :set_project_id

    private

    def set_project_id
      self.project_id = merge_request.source_project.id
    end