feat: add grpc health service

[hongalex]

What type of PR is this?

/kind feat

What this PR does / Why we need it:
Adds gRPC health check service to the grpc server.

Which issue(s) this PR fixes:
Closes #378

Special notes for your reviewer:
Replaces part of #381

[yuryu]

overall lgtm. Do you think it makes sense to add a simple test like

c := healthgrpc.NewHealthClient(conn)
got, err := c.Check(ctx, request)
if err != nil {
 // error
}
if got is not what we want {
  // error
}

[yuryu]

lgtm.

This is more like a wishlist rather than a request but I guess other tests should also use Run() and would also want to change Run() to accept a Listener instead of direct network address/port so that this test can use bufconn instead.

[yuryu]

suggest Errorf instead

https://github.com/golang/go/wiki/TestComments#keep-going

[yuryu]

ditto

[hongalex]

This is more like a wishlist rather than a request but I guess other tests should also use Run() and would also want to change Run() to accept a Listener instead of direct network address/port so that this test can use bufconn instead.

Agreed. I'll open an issue for this.