spec: certificates <> aggregate signatures?

[josef-widder]

We are now using certificates, i.e., sets of 2/3 vote messages, as first class data structure in consensus and other protocols.

I just wonder whether all the design and interfaces are ready for aggregate signatures, or whether there is something to think about and design (including the block structure)

Definition of done:

• Decision of whether we need to do something, and if so
• write an issue with concrete next steps (e.g., spec, adr, etc.)