Integration of osbuild composer with systemd

osbuild-composer now uses socket activation instead of hardcoded paths
in the code. osbuild-worker is an http client therefore it uses only
service unit. osbuild-worker must be started after the socket is
created. osbuild-composer service requires osbuild-worker to run, because without
it no jobs can be started.

osbuild-composer is executed as a regular user (newly created
_osbuild-composer user) as opposed to the worker which must run as root
in order to execute osbuild itself

distribution/osbuild-composer.conf

@@ -0,0 +1 @@
+u   _osbuild-composer   -   "OSBuild Composer user"