Provide implementations of `Permill`, `Perbill` etc in Subxt?

We fixed decoding into the generated Permill etc types a short while ago, but the types also have a bunch of custom logic and such. Currently, to work with them properly, one would have to pull in sp_arithmetic and convert the decoded parts into the proper types manually.

Perhaps we could either:

• Pull in sp-arithmetic and just substitute for the "proper" types. (sp-arithmetic doesn't look like it depends on much, so I think this would be fairly doable), or
• Make some concise implementations of the types ourselves in Subxt if pulling in the above is a pain for some reason. (I think we'd rather not do this)
• Leave things as they are and people will have to manually convert the types to use them.

Types are defined here: https://github.com/paritytech/polkadot-sdk/blob/master/substrate/primitives/arithmetic/src/per_things.rs#L1864

Start by reading the referenced per_things.rs definitions and comparing them with Subxt's generated Permill and Perbill decoding. Determine whether the intended outcome is adopting sp-arithmetic or maintaining concise Subxt implementations; done means the chosen types expose the needed custom logic without manual conversion.