[client] Fix - Add RPM metadata required for Red Hat software certification (#7562)

Declare the runtime dependencies, generate the changelog from git tags with
chglog at release time, and ship LICENSE, README.md and an example
/etc/sysconfig/netbird as %license, %doc and %config(noreplace). The unit
generated by "netbird service install" already reads that path via
EnvironmentFile, so post_install.sh is unchanged.

release_files/netbird.sysconfig

@@ -0,0 +1,14 @@
+# Environment for the netbird client service (netbird.service).
+# Every "netbird up" flag has an NB_* equivalent; see
+# https://docs.netbird.io/get-started/cli#environment-variables
+#
+# Red Hat Enterprise Linux 9 ships the in-kernel WireGuard module as a
+# Technology Preview and taints the kernel when it loads. For a fully
+# supported RHEL 9 configuration run the client with userspace WireGuard:
+#NB_WG_KERNEL_DISABLED=true
+#
+# Self-hosted management server:
+#NB_MANAGEMENT_URL=https://netbird.example.com
+#
+# Peer name shown in the dashboard (defaults to the system hostname):
+#NB_HOSTNAME=