dynamic dispatch fast_forward subcommand

[liamaharon]

Closes #12

• An InherentProvider with dynamic dispatch, heavily inspired by @pmikolajczyk41's static dispatch BlockBuildingInfoProvider in Substrate
• A new command fast-forward that takes a --chain arg, used to dynamically build an InherentProvider for the selected chain, also heavily inspired by @pmikolajczyk41's original fast-forward implementation in Substrate

TODO

• Get approach reviewed by @pmikolajczyk41 and also @kianenigma if he is available
• Decide if we should allow not passing --chain, in which case the cli will try all available inherent providers until it finds one that works (or doesn't)
• Consider seeding some more initial supported chains
• Write docs and doc comments