Roadmap for upstream filter from alpha to GA

*Title*: *Roadmap for upstream filter to GA*

*Description*:

The Upstream Filter (envoy.filters.http.upstream_filter) is currently marked as alpha, but it has become a critical dependency for the Envoy AI Gateway project and other advanced use cases.

The feature enables powerful post-router, pre-upstream HTTP filter processing, which is essential for implementing:
• External processing (ext_proc) logic after routing decisions
• AI workload orchestration (e.g., request transformation)
• Upstream authentication to cloud providers

Given its criticality for handling AI traffic, we ask for the roadmap for promoting the feature to GA.

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