feat(rtt): 'machine rtt' command to show round-trip time between macines usign using gossip data (#226)

* Add corrosion admin client function to get RTT to other machines in the cluster

* Add `uc machine rtt` showing all pair-wise RTT stats from corrosion

* Add long description to machine rtt command

* Include machine peer RTTs in InspectMachine instead of adding new gRPC API for it

* Add nil check on InspectMachine's Rtt field to protect from a potential edge case

* Generate docs for machine rtt command

* Handle m.Message being nil when a node is down or unavailble

* Update cli-docs

* Use tui Table instead of tabwriter

* Change reported RTT to the median rather than mean. Still calculate include stddev, as it might be a useful indication of network jitter.

* Show RTT to peers in `wg show` output

* Update cli docs

internal/machine/cluster/cluster.go

@@ -344,3 +344,8 @@ func (c *Cluster) RemoveMachine(ctx context.Context, req *pb.RemoveMachineReques
 
 	return &emptypb.Empty{}, nil
 }
+
+// MemberRTTs returns the average and standard deviation of round-trip times from this member to each cluster member.
+func (c *Cluster) MemberRTTs() ([]corrosion.MemberRTTStats, error) {
+	return c.corroAdmin.ClusterMemberRTTs()
+}