Add manage_threads entitlement for reactor.core (elastic#125037)

This adds the manage_threads entitlement for reactor.core as part of the 
azure-repository module. It looks like this is a requirement for offloading 
azure blob store work.

Author: jdconrad

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

@@ -14,3 +14,5 @@ com.azure.identity:
     - relative_path: "storage-azure/" #/config/storage-azure/azure-federated-token
       relative_to: config
       mode: read
+reactor.core:
+  - manage_threads