Create a fetch wrapper for off-chain services like

[stevenkhong]

Summary

dApps often interact with off-chain services (usually indexers) to retrieve historic chain state. This utility class will serve as foundation for building those query clients.

Goals

1. Provide basic functionalities for conveniently create query clients for services do not support protobuf.
2. Will be extended by Hydrogen and Insights clients.
3. Primary problem this wrapper should solve is how to easily add query endpoints and register types.

Non-goals

1. Create elaborate http handlers like axios, we can use a library for that.

Tasks

Create fetch client factory:

• Create fetch wrapper class in @demex-sdk/core
• Add example on how to use
• Update README.md

Create clients:

• Hydrogen Client
• Insights Client

References

Previous iterations of query clients:

• Hydrogen
• Insights

[stevenkhong]

done on #21. updated issue message to include TODO for Hydrogen and Insights fetch clients.