Design WorkerHeartbeat functionality

[liliankasem]

Create a design document to outline an approach for implementing the WorkerHeartbeat functionality. This should include the protobuf design and host implementation details.

WorkerHeartbeat will help us improve reliability as we can use this request to determine the state of the worker and use that information in turn to determine when to start/stop a worker process. The worker should send a heartbeat message every ~10 sec so that the host can then figure out health if pings did not come as expected. As of now host relies on worker_init_response - worker heartbeat can be extended to include rich set of information

Our protobuf currently has an empty definition for the WorkerHeartbeat here:

https://github.com/Azure/azure-functions-host/blob/dev/src/WebJobs.Script.Grpc/azure-functions-language-worker-protobuf/src/proto/FunctionRpc.proto#L156

[liliankasem]

Potentially related to #4852

There seems to be a disconnect between the host reading events and the out-of-proc worker being up and running and ready to handle invocations.

[fabiocav]

Moving this to the backlog for discussion, including potential issues with some worker's ability to implement this.

[fabiocav]

Discussion on this is in progress. Moving to 122 for completion.