Adds support for running the tunnel as a service on macOS via launchservices.
It also hooks up observability (`code tunnel service log`) on macOS and Linux.
On macOS--and later Windows, hence the manual implementation of `tail`--it
saves output to a log file and watches it. On Linux, it simply delegates to
journalctl.
The "tailing" is implemented via simple polling of the file size. I didn't want
to pull in a giant set of dependencies for inotify/kqueue/etc just for this use
case; performance when polling a single log file is not a huge concern.
Implements an automatic local download fallback, similar to SSH
(cc @roblourens). If the initial download results in an error, either
in making the request or a 5xx, it'll try to fall back to making the
request locally and streaming it over the tunnel.
This abstracts the request client behing a "SimpleHttp" trait which
either uses to the native reqwest or uses the 'delegated' mode over the
socket.
