refactor: Mirror substrate naming of TransactionValidity, ValidTransaction, and TransactionValidityError

## Issue summary

- The current code doesn't mirror the same names for associated data types within [`/primitives/runtime/src/transaction_validity.rs`](https://github.com/paritytech/substrate/blob/25388397aab20a24e2be55103dda95011aa04d7c/primitives/runtime/src/transaction_validity.rs)
- Implement `TransactionValidity` as `scale.Result`
- Implement `TransactionValidityError` as `scale.VaryingDataType`
- Rename current `TransactionValidity` to `ValidTransaction`
- Rename current `ValidTransaction` to `ExtrinsicValidTransaction`
- open to other suggestions
- Consider moving types to `lib/runtime` instead of `lib/runtime/transaction` to avoid stutter in the names.

## Other information and links

- https://github.com/paritytech/substrate/blob/25388397aab20a24e2be55103dda95011aa04d7c/primitives/runtime/src/transaction_validity.rs