[Draft] Support Multiple Backends in Proxy Mode

[fred-maussion]

This PR introduces the initial implementation for evolving proxymode to support fetching from multiple, parallel backends. The goal is to enable a highly-available and federated architecture, as presented in issue #1439.

This is a draft PR to gather early feedback on the core architectural changes.

Changes included in this draft:

• The configuration (ProxyOptions) is updated to accept a list of backend URLs with optional priorities.
• The core fetching logic (Search, Categories, Package) is refactored to query all backends in parallel using a fan-out/fan-in concurrency pattern.
• A shared, instrumented http.Transport is used for efficient connection pooling, while a new retryablehttp.Client is created per-request to correctly handle state (timeouts, retries, logging context).

Next Steps / To-Do:

• Add unit and integration tests for the new concurrency model.

Seeking feedback on the overall approach to parallel fetching and configuration parsing.

[prodsecmachine]

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[elasticmachine]

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