Pull base set of attributes into `BaseImporter`
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Work on this issue](https://contributors.gitlab.com/manage-issue?action=work&projectId=278964&issueIid=470486) - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=470486) </details> <!--IssueSummary end--> It might be worthwhile pulling a base set of attributes into the `BaseImporter` of `lib/gitlab/github_import/importer/events/..` to reduce the number of files changed if the events are updated in the codebase in future. see: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/157760#note_1979373781
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