[meta] `try-runtime-cli` `fast-forward`

[liamaharon]

Partial paritytech/polkadot-sdk#178

High-ish priority improvements to get fast-forward into a healthy state

• Support arbitrary inherents required by the runtime. ideas:
  • allow configuration of inherents using cli args (probably simplest, but least flexible and bad developer UX)
  • when try-runtime feature is enabled, add a special runtime api (or some other method) to expose some information about which basic inherents are expected by the runtime. this way, fast-forward can query that api when it boots up and know which inherents it needs to apply in its otherwise empty blocks (more complex but would support any runtime, seems like a good option if it's possible)
  • How I am going to approach solving this in the new try-runtime-cli repo:
    • Create a file inherents_configurations.rs containing inherent configurations similar to https://github.com/paritytech/substrate/blob/master/utils/frame/try-runtime/cli/src/block_building_info.rs
    • When a command like fast-forward that requires inherents is used, the CLI will attempt to build blocks using each of these inherent options until it finds one that works
    • If it finds one that works, it will memoize it for the rest of the run
    • If it cannot find one that works it exits with a helpful message to the user linking to documentation explaining how to add their inherent configuration to inherents_configurations.rs
• Needs documentation