cleoc: Set a custom User-Agent in the HTTP requests

> I also suggest sending the version string as the User-Agent so you can debug server-side issues. (Assuming your CLI uses an API of some sort)

https://medium.com/@jdxcode/12-factor-cli-apps-dd3c227a0e46

https://stackoverflow.com/questions/51628755/how-to-add-default-header-fields-from-http-client
https://stackoverflow.com/questions/2601372/what-is-the-standard-format-for-a-browsers-user-agent-string

One valid example would therefore be: `cleoc/0.1.0-dev+2023-01-02T150018Z.d2dfc51`

Start by locating cleoc's HTTP request entry points and how the application exposes its version. Use the issue's example as the target User-Agent shape, then verify that requests send the cleoc version and that the resulting behavior is covered by the relevant request tests, if present.

Written by the indexing model from the issue text.