[Datastore] expose 'reserveIds' API

[athakor]

Expose the 'reserveIds' API to Prevents the supplied keys' IDs from being auto-allocated by Cloud Datastore.

• reserveIds

/cc: @BenWhitehead

[BenWhitehead]

@athakor I think it makes sense to wire this through to mirror allocateIds. Do you have a proposed design of what this will look like?