Add missing entitlement to repository-azure

This entitlement is required, but only if validating the metadata
endpoint against `https://login.microsoft.com/` which isn't something we
can do in a test. Kind of a SDK bug, we should be using an existing
event loop rather than spawning threads randomly like this.

Author: DaveCTurner

modules/repository-azure/src/main/plugin-metadata/entitlement-policy.yaml

@@ -16,3 +16,5 @@ com.azure.identity:
       mode: read
 reactor.core:
   - manage_threads
+reactor.netty.core:
+  - manage_threads # https://github.com/elastic/elasticsearch/issues/128046