Figure out how to improve type safety for getBlockProduction API

[mcintyre94]

See: #1263 (comment)

Ideally we'd like this to be a type error:

const blockProduction = await rpc.getBlockProduction({identity: 'abc' as Base58EncodedAddress})
blockProduction.value.byIdentity['def' as Base58EncodedAddress]

This typescript playground shows how the types used should be achieving this

But in practice this isn't happening for the web3.js library. I'm guessing this is to do with the proxy indirection, but it needs more investigation.

[github-actions]

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