Add Kubernetes discovery e2e test helpers and adopt them in keda

Adds datadog_checks_dev/datadog_checks/dev/kube_discovery.py, a new module
providing Kubelet-Autodiscovery e2e test helpers. It deploys a sleeping,
RBAC-scoped Agent pod inside the integration's existing kind cluster and
execs `agent check` into it, so config-discovery can be exercised via real
Kubelet Autodiscovery without a full long-running in-cluster Agent.

Wires keda's conftest.py/test_e2e.py up to the new helpers, restoring
keda's previously-dropped test_e2e_discovery and adding
test_e2e_discovery_all_candidates.

Environment: Datadog workspace

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Co-authored-by: Vincent Whitchurch <vincent.whitchurch@datadoghq.com>

Author: vitkyrka