controller: Separate instance_client

[ggevay]

This is a refactoring related to the frontend peek sequencing, partially addressing the second paragraph of #33713 (comment) + other related discussions with Moritz and Jan.

This PR is purely code movement: it moves the Client from instance.rs into a new file instance_client.rs, and renames it to InstanceClient. (This is used by the frontend peek sequencing directly, bypassing the ComputeController, because we don't want the Coordinator to be a bottleneck for fast-path peeks.)

Part 1 of 3 from the original #34839. (Based on @teskje's comment there, we probably do want this code movement, right?)

[github-actions]

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