Remove usage of allowedConfigFeatures

[qreshi]

Description

Related common-utils PR: opensearch-project/common-utils#144

• Removes allowedConfigFeatures from the GetPluginFeaturesResponse to reflect the common-utils changes
• Removes allowedConfigFeatures from NotificationCore core interface in spi
• Removes the allowedConfigFeatures plugin setting

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

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[codecov-commenter]

Codecov Report

Merging #387 (1426c8b) into main (9f31061) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #387   +/-   ##
=======================================
  Coverage   86.40%   86.40%           
=======================================
  Files          51       51           
  Lines        1479     1479           
  Branches      352      352           
=======================================
  Hits         1278     1278           
  Misses        198      198           
  Partials        3        3           

Flag	Coverage Δ
dashboards-notifications	86.40% <ø> (ø)

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[adityaj1107]

Changes in common-utils for GetPluginFeaturesResponse merged here: opensearch-project/common-utils#144