`RuntimeGenesiConfig`: json macro added (#5813)

This PR adds `build_struct_json_patch` which helps to generate a JSON
used for preset.

Here is doc and example:

https://github.com/paritytech/polkadot-sdk/blob/d868b858/substrate/frame/support/src/generate_genesis_config.rs#L168-L266

And real-world usage:

https://github.com/paritytech/polkadot-sdk/blob/d868b858

/cumulus/parachains/runtimes/assets/asset-hub-rococo/src/genesis_config_presets.rs#L37-L61

Closes #5700

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Co-authored-by: Sebastian Kunert <[email protected]>