hi: add list-versions subcommand

Prints the tailscale versions integration tests use, with optional
filter and format flags. Source of truth stays in hscontrol/capver;
CI shells out to this instead of duplicating the version list in
YAML.

Examples:
  hi list-versions
  hi list-versions --set=must --exclude=head
  hi list-versions --set=all --format=json

cmd/hi/main.go

@@ -29,6 +29,13 @@ func main() {
 					return runDoctorCheck(env.Context())
 				},
 			},
+			{
+				Name:     "list-versions",
+				Help:     "Print Tailscale versions used by integration tests",
+				Usage:    "list-versions [flags]",
+				SetFlags: command.Flags(flax.MustBind, &listVersionsConfig),
+				Run:      listVersions,
+			},
 			{
 				Name: "clean",
 				Help: "Clean Docker resources",