Introduce PluginSubject and replace usages of stashContext in TransportGetAlertsAction

[cwperks]

Description

This PR replaces usages of ThreadContext.stashContext with a replacement that enforces strong ownership over system indices. The reporting plugin can access the system indices it declares with SystemIndexPlugin.getSystemIndexDescriptors, but otherwise needs to declare additional perms like this: https://github.com/opensearch-project/geospatial/blob/main/src/main/resources/plugin-additional-permissions.yml

The replacement restricts what plugins can do inside a privileged context, but permits access to their own system indices.

This PR also attempts to replace usages of scope.launch and client.suspendUntil with async code prevent blocking operations.

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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[cwperks]

Checks will fail until this repo is version bumped to 3.3