command, docs: create and document consul token configuration for connect acls (gh-6716)

This change provides an initial pass at setting up the configuration necessary to
enable use of Connect with Consul ACLs. Operators will be able to pass in a Consul
Token through `-consul-token` or `$CONSUL_TOKEN` in the `job run` and `job revert`
commands (similar to Vault tokens).

These values are not actually used yet in this changeset.

command/agent/testdata/basic.json

@@ -111,6 +111,7 @@
   "consul": [
     {
       "address": "127.0.0.1:9500",
+      "allow_unauthenticated": true,
       "auth": "username:pass",
       "auto_advertise": true,
       "ca_file": "/path/to/ca/file",