Refactor SSH init code, fix directory creation for TrustedUserCAKeys file (#20299) (#20306)

Backport #20299. Follow #20298. Only the `GlobalInitInstalled` function should prepare the SSH files for external server or starts the builtin server.
* `trustedUserCaKeys` is removed, use `SSH.TrustedUserCAKeys` directly
* introduce `ssh.Init`, move the SSH init code from `routers/init.go` to it
* `ssh.Init` will start builtin SSH server or prepare external SSH server files

modules/ssh/ssh_graceful.go

@@ -29,7 +29,7 @@ func listen(server *ssh.Server) {
 	log.Info("SSH Listener: %s Closed", server.Addr)
 }
 
-// Unused informs our cleanup routine that we will not be using a ssh port
-func Unused() {
+// builtinUnused informs our cleanup routine that we will not be using a ssh port
+func builtinUnused() {
 	graceful.GetManager().InformCleanup()
 }