[bwplotka]

Adds the discovery-kubernetes Cargo feature (["discovery", "http-config"]) with KubernetesSdConfig validation, build_pod_target_group matching Prometheus pod.go, and KubernetesPodDiscovery querying the Kubernetes REST API directly via reqwest without kube-rs or k8s-openapi. Part of prometheus/scrape.rs#36.

Stack:

1. discovery: add TargetGroup, StaticConfig, and Prometheus duration helpers #39 discovery: add TargetGroup, StaticConfig, and Prometheus duration helpers
2. discovery/dns: add DNS service discovery (dns_sd_configs) #43 discovery/dns: add DNS service discovery (dns_sd_configs)
3. http_config: add Prometheus HTTPClientConfig and reqwest client builder #44 http_config: add Prometheus HTTPClientConfig and reqwest client builder
4. discovery/kubernetes: add pod service discovery (role: pod) #45 discovery/kubernetes: add pod service discovery (role: pod) (this PR)

Commits in reading order:

1. 4ca6797: discovery/kubernetes: add KubernetesSdConfig and pod target group builder
2. b87ab0e: discovery/kubernetes: add reqwest-based KubernetesPodDiscovery runner

Intentional differences from Go

• KubernetesPodDiscovery queries Kubernetes REST list/get endpoints (pods, nodes, namespaces, replicasets, jobs) directly with reqwest instead of running client-go shared index informers as in discovery/kubernetes/kubernetes.go#L284 and discovery/kubernetes/pod.go#L63. Target group and __meta_kubernetes_* label construction in build_pod_target_group matches discovery/kubernetes/pod.go#L366.
• KubernetesSdConfig parses and validates all six Prometheus roles and kubeconfig_file in discovery/kubernetes/kubernetes.go#L108-L111. KubernetesPodDiscovery::new currently executes role: pod using explicit api_server or in-cluster environment and serviceaccount files in discovery/kubernetes/kubernetes.go#L307, and returns DiscoveryError::InvalidConfig when kubeconfig_file in discovery/kubernetes/kubernetes.go#L299-L300 or another role is requested.