rpc: Enable ChainSpec for polkadot-parachain (#5205)

This PR enables the `chainSpec_v1` class for the polkadot-parachian. 
The chainSpec is part of the rpc-v2 which is spec-ed at:
https://github.com/paritytech/json-rpc-interface-spec/blob/main/src/api/chainSpec.md.

This also paves the way for enabling a future `chainSpec_unstable_spec`
on all nodes.

Closes: https://github.com/paritytech/polkadot-sdk/issues/5191

cc @paritytech/subxt-team

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>