slot-based-collator: Extract signaling task

[skunert]

The slot based collator should have a separate signaling task that tells the block building task when to build a block.

The logic when to build can then be customized to allow different scenarios.
Some examples include:

• Just build each slot (status quo)
• Lookahead compatibility (Build each slot, build on all relay forks)
• Build multiple blocks per slot, but not directly after another (something like subslots)