servicemesh: proxylib image for the separated cilium-envoy image

[jrajahalme]

While #25081 allows cilium-envoy to run as a separate container, we still have to figure out how/if to pair a proxylib image (libcilium.so) with the external container. Proxylib uses a our stable Go extensions for Envoy API so that a Cilium agent build can add additional L7 parsers to Envoy without re-building the Envoy image. Cilium agent can then build control plane (e.g., policy) support for a given L7 protocol and ship an implementation of it as libcilium.so and configure Cilium Envoy filters to use it. These proxylib images are not useful by themselves, outside of them being used with Cilium. While we were shipping cilium-envoy within the Cilium agent image there was no problem with this, but now that we start supporting a separate cilium-envoy container image we need to figure out how to ship proxylib for it.

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[mhofstetter]

Fixed with cilium/proxy#232 & #26101