Add memory readings for Draytek OS (#15618)

Draytek use a custom OID to define their devices memory usage, defined here: https://www.draytek.com/support/knowledge-base/5517

This commit includes a mempool definition with the new memory OID `1.3.6.1.4.1.7367.3.7` and relevant LibreNMS tests

includes/definitions/discovery/draytek.yaml

@@ -1,3 +1,7 @@
 modules:
     os:
         sysDescr_regex: '/Router Model: (?<hardware>[\w ]+), Version: (?<version>[\w\.]+)/'
+    mempools:
+        data:
+            -
+                percent_used: .1.3.6.1.4.1.7367.3.7.0