nixos/freeradius : init - Added freeradius service
nixos/freeradius : init - Added freeradius service

Inspired from the dhcpd service implementation
Only 2 configurations options at the moment:
- enabled
- path to config directory (defaults to /etc/raddb)

Implementation was also inspired from ArchLinux
systemd file and corrected with @dotlambda and
@fpletz help.

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