Allow reuse client for external GRPC services

[mxpv]

In firecracker-containerd project we use GRPC plugins to extend containerd's functionality and we have to manage a separate client in order to make calls.
It would be great if we could reuse existing containerd's client (e.g. reuse connection object, namespace handling, reconnect logic, etc) to invoke external grpc services as well.

The proposed solution adds ExternalService and looks as follows:

	client, err := containerd.New(...)
	if err != nil {
		panic(err)
	}

	var firecrackerClient proto.FirecrackerClient
	client.ExternalService(func(conn *grpc.ClientConn) {
		firecrackerClient = proto.NewFirecrackerClient(conn)
	})

	firecrackerClient.XXX();

Signed-off-by: Maksym Pavlenko pavlenko.maksym@gmail.com

[codecov-io]

Codecov Report

Merging #3208 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3208   +/-   ##
=======================================
  Coverage   44.63%   44.63%           
=======================================
  Files         113      113           
  Lines       12161    12161           
=======================================
  Hits         5428     5428           
  Misses       5898     5898           
  Partials      835      835

Flag	Coverage Δ
#linux	48.65% <ø> (ø)	⬆️
#windows	39.86% <ø> (ø)	⬆️

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[crosbymichael]

I'd rather just export the Conn() on the client than make a public func like this. Can we do that?

[mxpv]

@crosbymichael that works too. Done.

[dmcgowan]

LGTM

[crosbymichael]

LGTM