Auto Config with custom policies

Please search the existing issues for relevant feature requests, and use the reaction feature (https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to add upvotes to pre-existing requests.

#### Feature Description

Assign custom policies to token generated with Auto Config.

Something like :

```
[…]
"auto_config": {
"enabled": true,
"intro_token_file": "/etc/consul/jwt",
+ "policy": "example",
"server_addresses": [
"192.0.2.1",
"192.0.2.2",
"192.0.2.3"
]
},
[…]
```

#### Use Case(s)

Being able to deploy a client node with an auto config token linked to a specific policy (with DNS capabilities for example)

Start by searching existing issues for related feature requests, as requested, then trace Consul's Auto Config handling and token policy assignment. The issue names no files or tests; done would mean supporting a custom policy in the auto_config configuration so the generated token is linked to that policy.

Written by the indexing model from the issue text.