Make kubernetes autodiscover and metadata work for other resources

[exekias]

As of today, both kubernetes autodiscover provider and add_kubernetes_metadata processor only work with Pods. We want to extend their support so we can do both autodiscover and enrichment based on other kinds of resources (for instance, nodes, services, namespaces...).

A configuration like this could be a good fit (up to discussion):

metricbeat.autodiscover:
  - providers: kubernetes
    resource: node
    ...

This would make autodiscover launch configurations based on cluster nodes.

About add_kubernetes_metadata, we need to figure out what's the best approach for the config

cc @vjsamuel @sorantis @odacremolbap @ChrsMark

[exekias]

One of the things that we could base on this: #13873