Implement Up & Downstream Connection of Blocks

[PaulKalho]

In the frontend, we can already connect the blocks.

The idea is, to save these connections in the frontend, and then to save the given configuration & the blocks coordinates on the workbench, as soon as the user clicks on the Save Icon in the top right.

Therefore, we need to think of a good schema to send to the backend, which configures the up and downstreams between blocks.

If you have any idea how this could be solved more elegant, feel free to discuss that here aswell.

Currently blocked by #91

We need to check in the frontend if the type of one input matches with the type of the incoming input. If this is not the case, a upstream/downstream should not be possible

This also includes the functionality to query the workflow from the backend to be shown on the workbench (upstreams/downstreams and coordinates)