[Lens] Provide a way to extends existing visualizations via a plugin-like API

[dej611]

Describe the feature:

It would be nice to let developers enrich current Lens visualizations with new custom features via a plugin-like API.
The API could let the developer declare new layer types for specific visualization types and expose it in the editor.
It could also offer a way to extends existing layer types.

The design of such API would be clear and defined and probably not cover everything, but allow enough freedom to the developer to extends and strong guarantees on the Lens side to avoid the editor to crash in case of integration issues.

[elasticmachine]

Pinging @elastic/kibana-visualizations @elastic/kibana-visualizations-external (Team:Visualizations)

[markov00]

In order to provide better transparency of priorities, issues that will not be prioritized within the next 24 months are being closed.

Tracking request in Lens general improvements ice box #184459