Extract exaudfclient-extension into dedicated repository

[tkilias]

Background:

• Currently, changes at the exaudfclient triggers unnecessary builds and tests on the flavors.
• Extracting it, also reduces the number of tests which need to run for building flavors, because the general tests for the functioning of the client already run it its repository, don't need to run for all changes to the flavors
• Further, it enforces test separation which means your tests test smaller parts

Tasks:

• Extract code to new repository
• Extract tests
• Create test setup for the new repository (includes run exaslct on specialized test language container flavors)
• Setup CI for new repository
• Create package management such that language container flavor can import it (either build it for certain language versions and release these or source packages and build it in the language container)
• Solve import for exaudfclient-base