OnlineIDE: Allow performant context switches between Theia Workspaces

[iyannsch]

Is your feature request related to a problem?

tba

Describe the solution you'd like

Make sure that the Artemis Server does not create new sessions when there are already existing ones. Also verify that Theia keeps up the sessions in sync with Artemis for a certain amount of time.

Describe alternatives you've considered

tba

Additional context

This feature relates to #8602

As a student, I want to switch between multiple Theia sessions without awaiting the initial loading delay again.

As a system administrator, I want to have mostly lightweight Theia containers running.

As a user, I want to start working and not wait for my online IDE to spin up.