Implement remote plugin search

[copyberry]

Implement remote plugin search

What changed

• Implement plugin/search by querying the remote plugin service without using the catalog cache.
• Support global, workspace, and personal scopes with bounded page sizes and passthrough cursors.
• Respect plugin feature gates and omit shared workspace results when plugin sharing is disabled.
• Keep search terms and pagination tokens out of transport errors and telemetry, and return search results as uninstalled plugin summaries.

Testing

• Add remote search coverage for request parameters, result conversion, authentication, pagination, scope mapping, and error redaction.
• Add app-server coverage for remote-plugin and plugin-sharing feature gates.

[github-actions]

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