feat: chain-api generates OpenRPC definitions

[aatifsyed]

Summary of changes

• chain_api function signatures expose argument arity
• chain_api functions are private
• chain_api::serve registers functions to a self-describing module

Other information and links

#4032

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.