Add StartResharding operation to update_collection_cluster REST API

[ffuugoo]

Tracked in #4213.

This PR adds StartResharding operation to update_collection_cluster REST API.

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[ffuugoo]

Rebased on dev. This is ready for final review, I think. StartResharding operation is not yet exposed in the OpenAPI spec (just in case we will cut a release before finalizing resharding).