[Backport 2.0] Add default roles for Notifications plugin

[opensearch-trigger-bot]

Backport 25eda95 from #1847

[codecov-commenter]

Codecov Report

Merging #1848 (c96ef30) into 2.0 (76092ff) will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.0    #1848      +/-   ##
============================================
+ Coverage     60.82%   60.85%   +0.03%     
- Complexity     3185     3187       +2     
============================================
  Files           253      253              
  Lines         17921    17921              
  Branches       3203     3203              
============================================
+ Hits          10900    10906       +6     
+ Misses         5444     5439       -5     
+ Partials       1577     1576       -1     

Impacted Files	Coverage Δ
...security/auditlog/sink/ExternalOpenSearchSink.java	61.72% <0.00%> (+2.46%)	⬆️
...nsearch/security/dlic/rest/api/AuditApiAction.java	68.08% <0.00%> (+4.25%)	⬆️
...ecurity/configuration/StaticResourceException.java	25.00% <0.00%> (+25.00%)	⬆️

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