Add missing entitlement to repository-azure (elastic#128047) (elastic#128059)

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

docs/changelog/128047.yaml

@@ -0,0 +1,6 @@
+pr: 128047
+summary: Add missing entitlement to `repository-azure`
+area: Snapshot/Restore
+type: bug
+issues:
+ - 128046

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