Future Work: Implement Finalized blockstream for Avail (for watchtower)

[tuommaki]

Watchtower should work only by finalized Avail blocks. There must be a separate Avail blockstream implementation for the watchtower use case, which uses SubscribeFinalizedHeads.

NOTE: That API has a caveat where it's notifications can jump over multiple blocks. In these cases, the blockstream implementation must fill in the gaps, as the in-between blocks are also assumed to be finalized.