[v0.1] Envoy ExtProc integration for Envoy AI Gateway

[rootfs]

Acceptance:

• ExtProc header/body mutation consistent with Envoy AI Gateway filter chains (documented setup for each)
• Example Envoy configs for common patterns (e.g., A/B test, canary routing)

[rootfs]

@Xunzhuo is it fixed by #245 ?

[Xunzhuo]

yes