Consul: agent config updates for WI (#18774)

This changeset makes two changes:
* Removes the `consul.use_identity` field from the agent configuration. This behavior is properly covered by the presence of `consul.service_identity` / `consul.task_identity` blocks.
* Adds a `consul.task_auth_method` and `consul.service_auth_method` fields to the agent configuration. This allows the cluster administrator to choose specific Consul Auth Method names for their environment, with a reasonable default.

command/agent/testdata/basic.hcl

@@ -245,13 +245,16 @@ consul {
   auto_advertise         = true
   checks_use_advertise   = true
   timeout                = "5s"
-  use_identity           = true
+  service_auth_method    = "nomad-workloads"
+  task_auth_method       = "nomad-tasks"
+
   service_identity {
     aud  = ["consul.io", "nomad.dev"]
     env  = false
     file = true
     ttl  = "1h"
   }
+
   task_identity {
     aud  = ["consul.io"]
     env  = true

command/agent/testdata/basic.json

@@ -167,6 +167,9 @@
       "server_rpc_check_name": "nomad-server-rpc-health-check",
       "server_serf_check_name": "nomad-server-serf-health-check",
       "server_service_name": "nomad",
+      "service_auth_method": "nomad-workloads",
+      "task_auth_method": "nomad-tasks",
+
       "service_identity": {
         "aud": [
           "consul.io",
@@ -187,7 +190,6 @@
       },
       "timeout": "5s",
       "token": "token1",
-      "use_identity": true,
       "verify_ssl": true
     }
   ],