`GenesisBuilder`: `arbitrary_precision` feature enabled for `serde_json` (#2987)

[`arbitrary_precision`](https://github.com/serde-rs/json/blob/6d44b9fac9269b4decf76acac5d68e8ec9d10c58/Cargo.toml#L69-L75)
feature allows to (de-)serialize big numbers w/o error.
For some details refer also to
https://github.com/paritytech/polkadot-sdk/pull/1256#discussion_r1455564450

fixes: #2963