Pull base set of attributes into `BaseImporter`
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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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