Use JSON + builder pattern in chain specs

[HCastano]

This Polkadot-SDK PR made a bunch of changes around how Rust-based chain specs were able to be constructred. In particular it introduced support for a JSON based approach to building the genesis configs used in the chain spec.

This makes the chain spec easier to read as it may only include fields which you want populated, leaving the omitted fields as Default's.

This feature was only released in Polkadot v1.4.0, so we can't take advantage of that feature until we upgrade.