feat: Add DiskConfig to instance selection to support disk config overrides for flexible VMs (#34704)

* feat: Add `DiskConfig` to instance selection to support disk config overrides for flexible VMs
docs: Update comments for disk configs and related fields

PiperOrigin-RevId: 941500907

Source-Link: googleapis/googleapis@282495b

Source-Link: googleapis/googleapis-gen@c056550
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