Add custom user-agent for Refresher client

[crystall1nedev]

This PR adds a custom user-agent for the Refresher app, closing #60.

The user-agent is built with Refresher/<autodiscover-version>. This was chosen over other options for a few reasons:

• This UA will be unique to Refresher, and servers can whitelist Refresher clients
  • Useful against repeated spam from bots and such
• Matching the autodiscover version can be useful server-side
  • Requests can be denied, for example, if something that is required is not available in an older version
    • v1 vs v2 is a great example of this!
• The user-agent only needs to be updated when the AutoDiscover version changes

[jvyden]

I probably won't implement any whitelisting/blacklisting/blocking/anything of the sort since I don't believe those limitations really need to exist, but this is indeed probably useful to someone to have. 😅