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