agones-{extensions,allocator}: Make servers context aware

[zmerlynn]

• adds an httpserver utility package to handle the Run function that controller/extensions use. Make that context aware using the same method as https.Run: https://github.com/googleforgames/agones/blob/dfa414e5e4da37798833bbf8c33919acb5f3c2ea/pkg/util/https/server.go#L127-L130
• also plumbs context-awareness through the allocator run{Mux,REST,GRPC} functions.
• adds a gRPC health server to the allocator, calls .Shutdown() on it during graceful termination - this seems to push the client off correctly.

Along the way, basically re-wrote the TestAllocatorAfterDeleteReplica test:

• moved the GetAllocatorClient up because it's fairly disruptive (new certs, etc.), so I wanted the pod stability checks to happen after
• changed agones-allocator stability checks to just grab the Deployment and wait for Replicas != AvailableReplicas.
• changed the test to actually disrupt the Pods in the middle of the test (vs before it just nukes the pods then starts the test.

Tested with e2e in a loop.

Towards #3853