Add version info command to signal server (#739)

Add version command to signal and management servers.

The version information will be filled during build time.

version/version.go

@@ -0,0 +1,9 @@
+package version
+
+// will be replaced with the release version when using goreleaser
+var version = "development"
+
+// NetbirdVersion returns the Netbird version
+func NetbirdVersion() string {
+	return version
+}