Implement building a fallback Kubernetes store snapshot

[czeslavo]

Based on the rejected Kubernetes store snapshot, its graph, and entity errors returned by the Gateway, this function/struct should generate a fallback Kubernetes store snapshot excluding all objects related to any object identified as invalid in the entity errors.

Note: this should implement the first step of the fallback configuration build algorithm that will not recover objects from their previous valid state.

Blocked by #5929.