Create ExecutionContext and show example with ActionPluginProxy

[cwperks]

Description

This PR creates a new part of the ThreadContext called ExecutionContext that is intended to hold an identifier to the plugin that is currently executing code. Ideally, this class is not exposed to plugins and is populated before the core defers execution to a plugin when an extension point is called directly and is cleared upon returning back to the core.

This PR does not consider cases where a plugin spins off a new thread or calls code in another plugin (is that possible?).

Related Issues

Related to: opensearch-project/security#4208

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• API changes companion pull request created.
• Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• Public documentation issue/PR created

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