Revisit custom RPCs in view of Metadata V15

[dmitrylavrenov]

As a follow up #828.

Details - paritytech/substrate#13272.

[dmitrylavrenov]

The issue is related to paritytech/substrate#12939.
Based on in it, to apply required changes we need to have our substrate fork update that includes:

• Metadata V15: Expose API to fetch metadata for version paritytech/substrate#13287
• Metadata V15: Add Runtime API metadata paritytech/substrate#13302
  At least, to polkadot-v0.9.42.