Introduce Heartbeat RPC

Current implementation of fleeting heartbeats does only test the connectivity of the instance. The fleeting plugins could also provide heartbeat RPC to check on infrastructure level the state of the instance.

e.g. check for

instance already terminated (spot instance interruption)
instance marked as unhealthy by the cloud provider
instance got already a interruption notice (spot instance)

See discussion here: gitlab-org/fleeting/plugins/aws!36 (comment 2423226925)

@ajwalker let's add the method already to the interface so plugins could start to implement the new RPC.

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