[7655] Upgrade GithubImport issue events mechanism

Firstly we tried to import issue event via request per each issue.
It consumed to much time to be done although imported all events.
Current upgrade uses single ednpoint per repository what
descreases number of requests.But it unfortunately also exclude
specificly cross-reference events.
So, we keep ability to choose between
- github_importer_single_endpoint_issue_events_import
  (that is expensive but full)
- github_importer_issue_events_import (should become default)
feature flags

See details:
https://gitlab.com/groups/gitlab-org/-/epics/7655
https://gitlab.com/gitlab-org/gitlab/-/issues/365977#note_1022018304