Add support for custom activity icons in v3

Is it possible to simply add icons to custom activities?
I searched the source code and found out that the `ActivityIconRegistry` is used to add icons but the `ActivityIconRegistry` is not exported therefore I can't import it inside my code.
Even if I can import the `ActivityIconRegistry` I'm not sure how to define the icon itself within my Angular application.

It would be really nice if I could set an `IconProvider` within the `Activity` attribute inside the backend.
This provider should be a class (same like the `OptionsProvider` from the `Input` attribute) with a specific interface which returns a svg icon as byte[] or string.
With this mechanism I don't have to add code to the frontend and can add an icon directly to the activity in the backend.

The `DefaultActivityTemplate` than simply has to check if the activity itself has an icon received from the backend and if not it should search inside the `ActivityIconRegistry` like it does for now.

Start by reading the existing ActivityIconRegistry and DefaultActivityTemplate usage, then trace how the Activity attribute and OptionsProvider expose backend-provided metadata to the Angular frontend. Define the required backend icon-provider interface and frontend fallback behavior; done means custom activities can supply SVG icon data from the backend while registry lookup still works when none is supplied.

Written by the indexing model from the issue text.