This replaces reqwest with the more simplistic ureq library for
performing required HTTP requests.
Reqwest comes with a lot of (tokio-based) machinery for
high-performance requesting that is a bit out of scope for
journaldriver's needs.
This clocks in at 62 (!) fewer dependencies after the change, with
equivalent functionality. Wew.
This implements the "meat" of the initial version of journaldriver.
Records from journald are converted into the representation required
by Stackdriver and forwarded to the API.
In this initial version journaldriver is only supported on instances
running on GCP.
