[canvas] Simplify and organize Canvas plugin lifecycle

[clintandrewhall]

• Related: Plugin system should fail if Setup lifecycle functionality is called after the setup lifecycle completes. #105439
• Related: [canvas] Reduce bundle by registering Canvas Plugin API on mount #104264
• Related: [RFC] Allow 'leasing' of Expression constructs #105164

Canvas appears to use setup dependencies in its mount function. Part of the problem here is not that Canvas is actually using setup dependencies, but that they're called setup dependencies, or we're using setup when we should be using either start dependencies or our own services.

Another issue is that setup, start and mount are all combined in a confusing way, obfuscating the true purpose of each portion of the lifecycle.

This issue will be considered complete when:

• Remove the internal CanvasSrcPlugin class in favor of services.
• The Canvas setup function accurately reflects setup tasks:
  • Create the ExpressionsService fork.
  • Create registries and return the "Canvas API", a set of functions which allow external plugins to add functions/renderers/etc to Canvas from their plugins.
  • Set the nav link/register with home/etc.
  • Create the mount function.
  • Nothing else.
• The Canvas start function accurately reflects start tasks:
  • Migrate legacy services that rely on setup dependencies to the new Service Abstraction architecture.
  • Start Canvas Services.
    • Provide the ExpressionsService fork to the expressions service.
  • Start the Loading Indicator.
  • Nothing else.
• The Canvas mount function accurately reflects mount activities:
  • Register renderers, functions, types, Canvas UI, etc with the registries created in setup.
  • Register Expression renderers, functions and types to the expressions service.
  • Render the application.

This will at least make the divide between the concerns much clearer to those looking at the code, but also avoid any non-compliance with #105439.

[elasticmachine]

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

[ppisljar]

registering renderers and expression function later than in setup phase will prevent you from doing reference extraction/injection and migration on your saved objects correctly.

This will need to be fixed before 8.0 due to change in saved objects id as else all existing workpads with expressions referencing other saved objects will break.

[ThomThomson]

Closing this for the time being as we aren't prioritizing Canvas tech debt currently.