feat: allow CurveJS init without RPC

[DanielSchiavini]

Let me be clear: I do not like throwing exceptions. Ideally we should make the invalid states unreachable. However, given the existing code base, throwing good errors seems like the only feasible way.

• CurveJS currently requires a web3 provider to be initialized
• However, we need a lot of data from CurveJS and we might not always have a RPC connection
• This PR allows users to retrieve (part of) the CurveJS data without requiring a RPC connection
• However, it will raise exceptions whenever a function that requires RPC is called

This PR is similar to curvefi/curve-stablecoin-js#26 and curvefi/curve-lending-js#34