Use pluginSubject for system index interaction and remove usages of stashContext

[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.

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 24 lines in your changes missing coverage. Please review.

Project coverage is 36.63%. Comparing base (f1ad865) to head (d69e965).

Files with missing lines	Patch %	Lines
...rch/jobscheduler/transport/RunAsSubjectClient.java	0.00%	19 Missing ⚠️
...rg/opensearch/jobscheduler/JobSchedulerPlugin.java	0.00%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #714      +/-   ##
============================================
- Coverage     37.67%   36.63%   -1.05%     
+ Complexity      135      133       -2     
============================================
  Files            22       23       +1     
  Lines          1189     1212      +23     
  Branches        109      109              
============================================
- Hits            448      444       -4     
- Misses          704      731      +27     
  Partials         37       37              

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