Config: Remove Index param

[jsdw]

One of the treickiest parts of getting started with Subxt is setting up the Config.

As a first step to simplifying it, can we remove the Index param? I believe it only exists so we know how to decode the value we get back from the runtime call, but I think we can be smarter and intelligently decode this value based on how many bytes it is. When we encode nonces for tx's it looks like we always encode to u64 anyway for that signed extension.