Introduce Membership type

[stas-demydiuk]

Replace strings with Membership type from this PR

Signed-off-by: Stanislav Demydiuk [email protected]

Checklist

• [x] Tests written for new code (and old code if feasible) - Not applicable
• [ ] Linter and other CI checks pass
• [ ] Sign-off given on the changes (see CONTRIBUTING.md)

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This change is marked as an internal change (Task), so will not be included in the changelog.