[DKG whiteboard] Restriction on broadcast

Context

The contract allows participating nodes to broadcast as many and as long data as they want. This issue is about adding restrictions to the messages sent on the whiteboard to avoid malicious behaviour:

• restrict the size of messages sent to the whiteboard.
• restrict the number of submissions per node if possible (to discuss with @jordanschalm).

Definition of Done:

• @tarakby

  • Once the format of broadcast messages is updated, compute the possible sizes of broadcast messages (verification vector/complaint/complaint answer)
  • Compute the number of broadcasts possible per node.

• @joshuahannan

  • rename the contract file to FlowDKGWhiteboard to avoid the confusion of thinking we're implementing DKG logic on chain. Renaming is not possible, a clarifying comment will be added instead.
  • update the contract to only accept certain sizes of broadcasts to the whiteboard (this is not about the size of final submissions which seems to be checked already)
  • [if confirmed] update the contract to accept a maximum number of broadcast per node per DGK instance.