[RFC] Blocks with Multiple Ports

[georg-schwarz]

Short Description

Modelling multiple ports in a blocktype would allow to implement features like a TableMergerBlock that take multiple inputs or produce multiple outputs.

Definition of Done

• The RFC document is created following the existing pattern
• A pull request with the RFC document is opened
• The pull request passed peer review and is accepted
• An issue is opened that tracks the implementation of the RFC

Note: The details of the RFC will be discussed in the PR, not in this issue!