fix: Set AdmissionRequest{}.RequestKind

[viccuad]

Description

Fix #148.

RequestKind matches the type of original API request, in the case that there's several kinds that match the same object (e.g: deployments can be modified via apps/v1 ad apps/v1beta1), and conversion was performed.

The AdmissionRequests use AdmissionRequest.Kind to match the rule of the webhook registered for, hence RequestKind is there for book-keeping.

Set RequestKind, which gets consumed later on in policy-server to set the metrics baggage:
https://github.com/kubewarden/policy-server/blob/63c9f32052b7ced50b4007e8ce0b5ed27392aa7f/src/worker.rs#L226

Test

Built an audit-scanner image with the fix and tested that kubewarden_policy_evaluations_total now contain resource_kind:

Before:

kubewarden_policy_evaluations_total{accepted="true", container="otc-container", endpoint="metrics", exported_job="unknown_service", instance="10.244.0.165:8080", job="policy-server-default", mutated="false", namespace="kubewarden", pod="policy-server-default-fd79dc9-8mf6b", policy_mode="monitor", policy_name="clusterwide-no-privileged-pod", request_origin="audit", resource_namespace="prometheus", resource_request_operation="CREATE", service="policy-server-default"}

After:

kubewarden_policy_evaluations_total{accepted="false", container="otc-container", endpoint="metrics", exported_job="unknown_service", instance="10.244.0.165:8080", job="policy-server-default", mutated="false", namespace="kubewarden", pod="policy-server-default-fd79dc9-8mf6b", policy_mode="monitor", policy_name="clusterwide-no-privileged-pod", request_origin="validate",
resource_kind="Pod",    <----
 resource_namespace="default", resource_request_operation="CREATE", service="policy-server-default"}

Didn't add unit tests, as there's not much to do there. Integration tests would be too much.

Additional Information

Tradeoff

Potential improvement