Use Abortable for async tasks in the blend service

[youngjoon-lee]

We have a dead code that stores a JoinHandle not used anywhere.
https://github.com/logos-co/nomos-node/blob/4c0d5168e6757e8c6aa5dd2b3e9160f0edea323c/nomos-services/blend/src/backends/libp2p.rs#L28-L30

Originally, it was for a later use for adding Abortable. That has been added to the DA code. But I haven't added it to the blend code yet. I should have added a TODO comment at least.

First of all, I should check if it's really necessary to add Abortable to the blend code. If so, I'll add it, as done in DA.