Allow block sources other than Oura

[SebastienGllmt]

Right now Carp uses Oura to fetch the block cbor. However, there is no reason that things should be hard-coded to Oura. You could use any tool that gets you the block cbor such as maybe scrolls or something else that may fetch blocks faster.

One way we could do this is by adding a tower implementation for Oura so that Carp can use any task that implements this trait. Doing this, however, probably depends on solving https://github.com/dcSpark/carp/issues/34#issuecomment-1140540177 first