API: Add streaming case for triggering a collection module

In an internal discussion, we decided that the collection modules are as stateless as possible. I.e. they are only working when the requirements/collection manager (which one is in charge we still have to decide) triggers a single collection. The manager is responsible for adhering to the intervals.

For better performance, a gRPC stream from manager to a collection module is set up; s.t. the streaming channel can be used for each new trigger of a collection. Instead of setting up a connection for each single collection which would be less efficient.