refactor `reqwest` usage to common helper

[recanman]

I think we probably want to refactor our use of reqwest a bit to set user-agent and default timeouts everywhere (ntfy, updates, webdav). i.e. single factory function for Client that is used everywhere. I think if you do the refactor in this PR it's fine and definitely apprechiated, otherwise just copy what ntfy does

Originally posted by @untitaker in #1054 (comment)