feat: Add a new fleet registration feature

Users can now register their clusters to a fleet by setting cluster.fleet.project. Please see https://cloud.google.com/anthos/fleet-management/docs/fleet-concepts for more details about fleet management.

PiperOrigin-RevId: 520023539

Source-Link: googleapis/googleapis@0fac9e8

Source-Link: googleapis/googleapis-gen@2319a88
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