Extend snapshot cache with custom resource types

while working on https://github.com/envoyproxy/go-control-plane/pull/1467 as alternative solution i wanted to propose to be able to register custom resource types in Snapshot cache. Nowadays, validation fails with unknown resource type when trying to generate snapshot with custom resource type. Am aware there exists TypedExtensionConfig, but that would require data plane changes and parsing typed extension config in data plane and even potentially using ecds. This suggestion would make snapshot cache more flexible and easier for users to migrate to use go control plane.cc @valerian-roche @agrawroh