add "index single block" functionality to voyager

[benluelo]

we currently have the FetchBlocks message in voyager, which is picked up by plugins (cosmos, ethereum, aptos) and exposed through the cli via the init-fetch subcommand.

add a new Call variant FetchBlock (notably singular) for fetching a single block, add it to all event source plugins (most already have a message for fetching a single block, if not all), and expose it through the cli.

[Nguyen-VanChi]

nice voyager

[aeryz]

TBH this is a very nice and clear issue. Maybe we can delegate this to a wannabe open-source contributor?

[benluelo]

implemented in #4699