Extend snapshot cache with custom resource types

[nezdolik]

while working on #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

[markdroth]

FWIW, I am very supportive of this from an xDS API shepherd perspective. In general, we want to be able to easily add new resource types when we want to, and any new resource type should just automatically work in ADS without needing to add a new RPC service for each resource type. It would be great if go-control-plane supported this model.

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[markdroth]

This is still desirable.

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[markdroth]

Still desirable. Can we at least mark this as non-stale?

[github-actions]

This issue has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in the next 7 days unless it is tagged "help wanted" or "no stalebot" or other activity occurs. Thank you for your contributions.

[markdroth]

Still needed.

[alecholmez]

no stalebot