Make `context` easily available in Plugins

[MaxTru]

Is your feature request related to a problem? Please describe.

In the current situation, there is no clear best-practice way of how to access the current modeler context from within a plugin.

Example: the ElementTemplatesModal requires access to the elementTemplates service via the active modeler instance of the active tab. There is no clear apparent way of how to do that (see analysis on this board: https://miro.com/app/board/o9J_lTaSpoI=/).

This leads to:

• Effort for developers who are trying to access context from within a plugin
• Potential sub-optimal solutions, since devs would always try to find a new good way to solve this issue (e.g., see the current elementTemplates solution, which is implemented in a way that element templates are re-validated although this has been done by the element template registry already)

Describe the solution you'd like

There should be a best-practice public API mechanism to get access to the current modeler instance from within a Camunda Modeler Plugin / React component.

We need to:

• [ ] Decide how to design the public API. We should consider an RfC document for this. For example something like:
• [ ] Decide how to internally implement this (e.g., by subscribing to modeler.created or modeler.updated) events

Describe alternatives you've considered

Additional context

See https://miro.com/app/board/o9J_lTaSpoI=/

[pinussilvestrus]

We recently implemented something similar in properties-panel-next.

• Provide a context on the root (in this example maybe the PluginRoot)
• Add a useService hook to easily fetch didi like services
• Use that one inside a Plugin

function MyPlugin(props) {
  const elementTemplates = useService('elementTemplates');

  // do stuff ...
}