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
