Incorporate stabilized APIs

The functions we test in `unstable_rpc_api_calls.rs` were stabilized:
https://github.com/paritytech/polkadot-sdk/issues/1156
https://github.com/paritytech/polkadot-sdk/issues/1508

We should provide these functions as actual API functions.