Add OncallScheduleEntity with data to members

What does this MR do?

This adds OncallSchedule information to the MemberUser entity, which will be used to warn users that the member is part of an Oncall Rotation when removing the user from a project.

See #323627 (closed) for more information on the feature.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Security

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Related to #323627 (closed)