OnlineIDE: Add dynamic plugin support for Online IDEs

[iyannsch]

Is your feature request related to a problem?

tba

Describe the solution you'd like

In Artemis, instructors should have the option to specify not only the Theia base image but also a set a list of plugins they want to have enabled in the Theia instance of their students.

Describe alternatives you've considered

tba

Additional context

This issue relates to #8602

As an instructor, I want to add plugins of my choice to the online IDE of my course. They should be preconfigured for all my students.

As a user, I want to have all suggested features automatically set up for me so that I don't need to worry about them.

As an instructor, I want to characterize my students' working environment so that it blends into my exercise. For example, I want to create a system without a debugger or low RAM resembling a resource-constrained system.

As a user, I want my code to pass online Artemis tests. Thus, I want my online IDE to be as constrained as the tests will be.

[iyannsch]

Not feasible to trigger the image build from Artemis with dynamic plugin addition. Buildtimes of >30min also make this project unfeasible for now.