CLI: enrich User-Agent when making API requests

While troubleshooting an unrelated issue, we saw that the User-Agent set by the CLI was simply `Go-http-client/1.1`. 

We can enrich this with more information, such as:
* Coder username/email
* CLI subcommand (e.g. `coder ssh`)
* Coder version

This should help platform administrators to track down where API requests are coming from.