Make own version of RuntimeVersion to avoid mismatches

[jsdw]

This came about because stateVersion went away in a recentish version of substrate/polkadot, which started breaking subxt which was quite strict in what it expected to be decoded.

Here we make the decoding more lenient, only expecting the props we need and keeping props that exist but aren't needed in a HashMap incase they are useful to others.

[maciejhirsz]

LGTM, if I were (more) pedantic I'd suggest testing if serialize works too, but at this point it would be just testing serde_json.

[dvdplm]

I don't think we ever serialize our flavour of RuntimeVersion right? We want to flatten when deserializing, but perhaps it would be safer to keep the other key when serializing a subxt::RuntimeVersion but not sure if that's a thing in serde.

[jsdw]

Good points; I'm not actually aware of any need to Serialize that on the subxt side so I might just remove that bound and avoid thinking about it :)

Others can easily replicate and serialize what they need anyway, and we can think about adding it back if it's genuinely useful.