This commit refactors the way how configuration files are deployed to
the `/etc/asterisk` directory.
The current solution builds a Nix derivation containing all config files
and symlinks it to `/etc/asterisk`. The problem with that approach is
that it is not possible to provide additional configuration that should
not be written to the Nix store, i.e. files containing credentials.
The proposed solution changes the creation of configuration files so
that each configuration file gets symlinked to `/etc/asterisk`
individually so that it becomes possible to provide additional config
files to `/etc/asterisk` as well.
917be9fa32