Add a new subcommand for indexing the chain

[elmattic]

          Note that the initial step of populating the index is still performed. In a future PR, we aim to remove it entirely and provide the user full control of the process via a subcommand:

forest index validate-backfill --from<epoch> --to <epoch>

This approach is similar to the one used by Lotus.

Originally posted by @elmattic in #5341 (comment)

[LesnyRumcajs]

Let's have a more fleshed out description for this subcommand.