frontend Map: Allow plugins to customize the map

[sniok]

Basic Map extension

This PR introduces 3 new functions that allow for the simplest case of extending the Map.

You can read the full docs page over here https://github.com/headlamp-k8s/headlamp/blob/d02531292dd770360d127a792d345e82428ad994/docs/development/plugins/functionality/extending-the-map.md

You can add your own nodes and edges, customize the icon for the nodes and show a details page for the node.

1. registerMapSource
   registers a "Source" which provides Nodes and Edges to the map

2. registerKindIcon
   adds an icon for a specific resource "kind", which is displayed on the node and the map search (not the global one)

3. registerKindDetailsPage
   registers details panel for a specific resource "kind"

Reasoning and intentions

The registerKindIcon and registerKindDetailsPage don't mention Map and have a generic name (and not registerMapNodeIcon for example) for a reason.
I think in the near future this can be used in other places and not just Map.
For example KindIcon can be used in the global search, and Details component can be used in the normal Details page for custom resources.
For now the customizations are very granular to keep it simple but I think in the future we should have an ability to register a custom resource that will integrate it everywhere all at once, something like this:

registerCustomResource(params) {
  registerKindIcon(...);
  registerKindDetailsPage(...);
  registerMapSource(...);
  registerGlobalSearch(...);
  registerSidebarItem(...);
  registerRoute(...);
}

which I think would be extremely useful since a lot of plugins are listing/displaying custom resources.