Add change stream new record models and tests.

[changliiu]

Include PartitionStartRecord, PartitionEndRecord, PartitionEventRecord, MoveInEvent, MoveOutEvent. These will be used in the following PRs - the returned change stream record will be parsed into these models, and corresponding actions will be added and called later.

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[changliiu]

assign set of reviewers

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[changliiu]

Friendly ping :)

[changliiu]

Friendly ping :)