…ealth check

* 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.

Tested with e2e in a loop.

Towards googleforgames#3853

…AllocatorClient