jsonnet: add separated kubernetes mixin and custom kube prometheus alerts

example.jsonnet

jsonnet/kube-prometheus/kube-prometheus.libsonnet

jsonnet/kube-prometheus/mixin/alerts/alerts.libsonnet

jsonnet/kube-prometheus/mixin/alerts/general.libsonnet

jsonnet/kube-prometheus/mixin/alerts/node.libsonnet

jsonnet/kube-prometheus/mixin/alerts/tests.yaml

jsonnet/kube-prometheus/mixin/custom.libsonnet

jsonnet/kube-prometheus/mixin/kubernetes.libsonnet

jsonnet/kube-prometheus/mixin/rules/general.libsonnet

jsonnet/kube-prometheus/mixin/rules/node-rules.libsonnet

jsonnet/kube-prometheus/mixin/rules/rules.libsonnet