"consul agent" should support -token-file

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#### Feature Description

The `consul login` command has the ability to write tokens to files, namely the `-token-sink-file` option. It would be nice if the `consul agent` command would get a `-token-file` option (incl. the CONSUL_HTTP_TOKEN_FILE env var) to make those two commands integrate nicely.

#### Use Case(s)

Generating a token and then using it requires to manually or using 3rd party tools (e.g. jq) to produce valid client config files or hcl fragments.