In the internal deployment, we want to talk with composer over a http/https
proxy. This proxy adds new composer.proxy field to the worker config that
causes the worker to connect to composer and the oauth server using
a specified proxy.
NB: The proxy is not supported when connection to composer via unix sockets.
For testing this, I added a small HTTP proxy implementation, pls don't
use this in production, it's just good enough for tests.
Signed-off-by: Ondřej Budai <ondrej@budai.cz>
