Replace usages of ThreadContext.stashContext with PluginClient

[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.

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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