subxt-codegen: Hide complexity and provide "simple" interface mirroring CLI/macro usage

[jsdw]

The subxt-codegen crate is a little complex to use, and it's easy to generate code that isn't valid (eg see #1211). Further, it's not really been given any TLC and isn't really code we'd want to support in a public interface anyway.

Let's hide away most of the complexirty and provide a builder-like interface that generates code with the same sort of options that the CLI tool or #[subxt] provides. The macro/CLI should both be able to use only this simple interface to do what they want.

We shouldn't cater for uses outside of generating code for Subxt here; A later step would be to factor the type generation stuff out into a separate crate so that others can generate rust types from metadata in a more general way than just for Subxt.

[jsdw]

Closed by #1225