Derive scale_info::TypeInfo for types used in polkadot

[ascjones]

Companion for paritytech/polkadot#3336.

[ascjones]

Hey @tomusdrw what's the process for merging "companion" PRs here, which depend on a substrate PR? Specifically paritytech/substrate#8615.

That PRs companion polkadot PR depends on this, which in turn depends on the substrate PR (sp-core).

[tomusdrw]

There is no clear process yet, until the companion system is expanded to this repo. I see couple options though:

1. Split sp-core changes into a separate substrate PR, so that we can merge this one easily.
2. Merge this PR anticipating quick merge of both Substrate & Polkadot companions (this would render our repo's CI broken for a brief period)
3. Merge Substrate PR with companion failing and then merge this PR.
4. Add temporary patch to polkadot companion to use aj-metadata-vnext branch for BEEFY components to make the CI green. Then either merge this and revert or merge polkadot and merge this & revert polkadot patch later.

TBH I'm most in favour of option (4), but it requires a bit more work on your side. However I'm going to be happy with any of these - excited to see this stuff in.