[Maps] Define and document a public maps plugin API for creating custom sources and layers

[maihde]

Describe the feature:

Kibana supports a plugin-system, but the maps sub-system does not expose a mechanism for 3rd party plugins to officially extend the map with custom sources or layers. I would like to request an official supported API within the maps subsystem that allows plugins to extend the map with new sources and layers.

Describe a specific use case for the feature:

We have a custom tile-map-server that provides additional features beyond a generic TMS (https://github.com/spectriclabs/elastic_datashader). We have developed tight integration within Kibana by making a custom-map-source available, but this is accomplished by directly modifying the Kibana source code (highly undesirable). Furthermore, making such modifications to Kibana could in some situations be considered in violation of section 2 of the Elastic License (also highly undesirable).

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[nreese]

@maihde We are working on providing a registry for sources and layer creation wizards. Check out PR #60870 and please provide feedback.

This PR adds a source_registry so plugins can register sources and the PR adds a layer_wizard_registry so plugins can add menu options to add layer list. There is more coming in this space but we are hoping to build on these registries. Let us know if this would provide the interfaces needed for your custom source.

[elasticmachine]

Pinging @elastic/kibana-presentation (Team:Presentation)

[nreese]

Closing, completed in #136761. That PR added an example of how to add a custom source