Add CLI command to get power table by tipset key / epoch

[masih]

During passive testing when the network name changes we can no longer use cert exchange, nor lotus F3 apis to fetch the power table corresponding to an instance from previous network names.

This is because on re-bootstrap the certstore is re-created with a new datastore namespace. Lotus APIs also will only function for current network.

But technically speaking, if we know the finalised epochs in the previous networks (e.g. via the observer server) we can fetch the power table that they used.
This would be a super handy utility to have, specially when debugging cases after a passive testing session is paused.

This utility will live in lotus f3 cli.