feat: add compact placement feature for node pools

Use a compact placement policy to specify that nodes within the node pool should be placed in closer physical proximity to each other within a zone. Having nodes closer to each other can reduce network latency between nodes, which can be useful for tightly-coupled batch workloads.

PiperOrigin-RevId: 488490422

Source-Link: googleapis/googleapis@452324b

Source-Link: googleapis/googleapis-gen@008d00f
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