config: add TTL to agent identity config (#18457)

Add support for identity token TTL in agent configuration fields such as
Consul `service_identity` and `template_identity`.

Co-authored-by: Michael Schurter <mschurter@hashicorp.com>

command/agent/testdata/basic.hcl

@@ -248,11 +248,13 @@ consul {
     aud  = ["consul.io", "nomad.dev"]
     env  = false
     file = true
+    ttl  = "1h"
   }
   template_identity {
     aud  = ["consul.io"]
     env  = true
     file = false
+    ttl  = "2h"
   }
 }