rpc: add `rpc v2 chainSpec` to polkadot (#2859)
The [chainSpec RPC API from the v2 spec](https://paritytech.github.io/json-rpc-interface-spec/api/chainSpec.html) was only added to substrate-node and should be added to polkadot as well /cc @lexnv
The [chainSpec RPC API from the v2 spec](https://paritytech.github.io/json-rpc-interface-spec/api/chainSpec.html) was only added to substrate-node and should be added to polkadot as well /cc @lexnv