slot-based-collator: Move spawning of the futures

[bkchr]

Move spawning of the slot-based collator into the run function. Also the tasks are being spawned as blocking task and not just as normal tasks.

[paritytech-workflow-stopper[bot]]

All GitHub workflows were cancelled due to failure one of the required jobs. Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/11937246140 Failed job name: test-linux-stable

[bkchr]

/cmd prdoc --audience node_dev --bump major