Replace usages of ThreadContext.stashContext with pluginSubject.runAs

[cwperks]

Description

This PR removes direct usages of ThreadContext.stashContext and implements a new extension point in core (IdentityAwarePlugin) which assigns this plugin a subject to use for the purposes of executing transport actions outside of the authenticated user context.

With this subject, job-scheduler is permitted to access its own system indices without having to use ThreadContext.stashContext explicitly.

Related Issues

Resolves opensearch-project/opensearch-plugins#238

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[codecov]

Codecov Report

❌ Patch coverage is 70.35775% with 174 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.08%. Comparing base (aeb2d9c) to head (7390f92).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
.../action/explain/TransportExplainTransformAction.kt	64.70%	21 Missing and 3 partials ⚠️
...lup/action/explain/TransportExplainRollupAction.kt	55.55%	20 Missing ⚠️
...transport/action/explain/TransportExplainAction.kt	67.39%	13 Missing and 2 partials ⚠️
.../rollup/action/start/TransportStartRollupAction.kt	58.62%	9 Missing and 3 partials ⚠️
...ransform/action/get/TransportGetTransformAction.kt	57.14%	10 Missing and 2 partials ⚠️
...ent/rollup/action/get/TransportGetRollupsAction.kt	50.00%	8 Missing and 2 partials ⚠️
...nt/rollup/action/stop/TransportStopRollupAction.kt	61.53%	8 Missing and 2 partials ⚠️
...form/action/start/TransportStartTransformAction.kt	68.96%	7 Missing and 2 partials ⚠️
...ment/rollup/action/get/TransportGetRollupAction.kt	63.63%	6 Missing and 2 partials ⚠️
.../notification/filter/NotificationActionListener.kt	50.00%	7 Missing ⚠️
... and 14 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1438      +/-   ##
==========================================
- Coverage   76.31%   76.08%   -0.23%     
==========================================
  Files         375      376       +1     
  Lines       18772    18763       -9     
  Branches     2366     2365       -1     
==========================================
- Hits        14326    14276      -50     
- Misses       3212     3253      +41     
  Partials     1234     1234              

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