Handle `is_deleted` attribute when querying for identities in ClickHouse

[matthewelwell]

Scope

1. Modify clickhouse schema to add is_deleted boolean attribute to the identities table
2. Modify business logic reading from clickhouse either by an additional clause, a row policy or a materialised view

Acceptance Criteria

• Rows can be written to the identities table with is_deleted attribute
• Rows with is_deleted: true do not appear in segment membership views