Chain API subsystem

## Issue summary

- The Chain API subsystem is responsible for providing a single point of access to chain state data via a set of pre-determined queries.
- Some extra research is required in term of whether we will need this subsystem. This is an implementation detail. As we move towards finishing more subsystems, we would make a call about whether we need this or no.

## Other information and links

- https://paritytech.github.io/polkadot-sdk/book/node/utility/chain-api.html

Start by reading the linked Chain API design document and compare its stated responsibilities with the subsystems already present in Gossamer. The issue explicitly calls for research before implementation; done means documenting whether this subsystem is needed and, if so, defining its scope.

Written by the indexing model from the issue text.