Introduce path prefix for generated ::subxt code

[cmichi]

I'm running into a problem using subxt-codegen:

• subxt-codegen generates code like this for certain code paths: ::subxt::tx::StaticTxPayload::new.
• If subxt is not a root dependency of the invoking crate this results in a compiler error could not find 'subxt' in the list of imported crates.

I suggest to introduce an optional prefix which can be passed to subxt-codegen.

[jsdw]

That makes sense, and it's the same sort of thing other proc macro crates in the ecosystem do to tackle the issue :)