Allow setting agent acl token with an env variable.

#### Feature Description
Currently the only way to set agent's acl token is via the config - [ref](https://developer.hashicorp.com/consul/docs/agent/config/config-files#acl_tokens_agent)
It would be helpful if this can taken from env variable (or update docs if it is possible even now).

#### Use Case(s)
Security wrt file permissions - setting it in .env file with different file perms that config file with different perms.

#### Precedent
This is already done in some places (read consul / nomad's config) requiring vault token. The docs prefer setting VAULT_TOKEN instead of in the .hcl / .json file.

The issue names the agent ACL token configuration field and references Consul, Nomad, and Vault, but no source file or test. Start by checking how agent ACL tokens and environment variables are currently handled, then determine whether behavior or documentation needs updating. Done means the requested environment-variable path is supported and documented, or existing support is documented clearly.

Written by the indexing model from the issue text.