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/api/pb/machine.proto

@@ -83,6 +83,8 @@ message MachineDetails {
   MachineInfo machine = 2;
   // Current Corrosion cr-sqlite database version (Lamport timestamp) of the cluster store.
   int64 store_db_version = 3;
+  // Round-trip times to other machines in the cluster, keyed by peer machine ID.
+  map<string, RTTStats> rtts = 4;
 }
 
 message TokenResponse {
@@ -129,3 +131,8 @@ message WireGuardPeer {
   int64 transmit_bytes = 5;
   repeated string allowed_ips = 6;
 }
+
+message RTTStats {
+  double median = 1;
+  double std_dev = 2;
+}