Add UUID and server info to introspection

Closes #1862

This adds a new rpc to the introspection service to provide server
information with a generated UUID that is done on demand and the os and
arch of the server.

ctr output:

```bash
> sudo ctr version

Client:
  Version:  v1.2.0-802-g57821695.m
  Revision: 578216950de9c1c188708369e2a31ac6c494dfee.m

Server:
  Version:  v1.2.0-802-g57821695.m
  Revision: 578216950de9c1c188708369e2a31ac6c494dfee.m
  UUID: 92e982a9-f13e-4a2c-9032-e69b27fed454
  OS: linux
  ARCH: amd64
```

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

client.go

@@ -57,6 +57,7 @@ import (
 	"github.com/containerd/containerd/snapshots"
 	snproxy "github.com/containerd/containerd/snapshots/proxy"
 	"github.com/containerd/typeurl"
+	"github.com/gogo/protobuf/types"
 	ptypes "github.com/gogo/protobuf/types"
 	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	specs "github.com/opencontainers/runtime-spec/specs-go"
@@ -678,6 +679,27 @@ func (c *Client) Version(ctx context.Context) (Version, error) {
 	}, nil
 }
 
+type ServerInfo struct {
+	UUID string
+}
+
+func (c *Client) Server(ctx context.Context) (ServerInfo, error) {
+	c.connMu.Lock()
+	if c.conn == nil {
+		c.connMu.Unlock()
+		return ServerInfo{}, errors.Wrap(errdefs.ErrUnavailable, "no grpc connection available")
+	}
+	c.connMu.Unlock()
+
+	response, err := c.IntrospectionService().Server(ctx, &types.Empty{})
+	if err != nil {
+		return ServerInfo{}, err
+	}
+	return ServerInfo{
+		UUID: response.UUID,
+	}, nil
+}
+
 func (c *Client) resolveSnapshotterName(ctx context.Context, name string) (string, error) {
 	if name == "" {
 		label, err := c.GetLabel(ctx, defaults.DefaultSnapshotterNSLabel)