Introduce a test to reproduce conversion webhooks on 0.15

[mimowo]

What would you like to be added:

An automated test which would reproduce the issues with conversion webhooks at scale encountered in 0.15

See the scenario: kubernetes/kubernetes#136950 (comment)

Why is this needed:

To help to guide the graduation of the ConcurrentWatchObjectDecode feature gate in k8s core: kubernetes/kubernetes#136950

[mimowo]

@tenzen-y @gabesaba @IrvingMg cc @mbobrovskyi

[mbobrovskyi]

/assign