chain-spec-builder: Do not swallow unknown fields

[skunert]

Follow up of #4685.

We should not swallow any fields when modifying existing chain-specs. Currently para_id and relaychain fields would not be present in the output chain spec.

[CrackTheCode016]

I believe the cause is due to the existing chain spec being parsed via ChainSpec::from_json, which assumes no extensions / custom fields, aka just the default config.

To ensure any fields can be preserved, we can simply parse the file via fs::read, then serde_json::read_slice, merge in place, versus converting to ChainSpec. Other option is to hardcode them as part of the default config but not sure we want that.

[bkchr]

Or we could just keep the original json file around, continue using from_json and when the operation is finished we merge back the new chainspec to the original file.