config: remove consul.template_identity in lieu of task_identity (#18540)

The original thinking for Workload Identity integration with Consul and Vault
was that we'd allow `template` blocks to specify their own identity. But because
the login to Consul/Vault to get tokens happens at the task level, this would
involve making the `template` block a new WID watcher on its own rather than
using the Consul and Vault hooks we're building at the group/task level.

So it doesn't make sense to have separate identities for individual `template`
blocks rather than at the level of tasks. Update the agent configuration to
rename the `template_identity` to the more accurate `task_identity`, which will
be used for any non-service hooks (just `template` today).

Update the implicit identities job mutation hook to create the identity we'll
need as well.

command/agent/testdata/basic.hcl

@@ -250,7 +250,7 @@ consul {
     file = true
     ttl  = "1h"
   }
-  template_identity {
+  task_identity {
     aud  = ["consul.io"]
     env  = true
     file = false