feat: allow 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.

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

This PR is similar to https://github.com/curvefi/curve-js/pull/394 and https://github.com/curvefi/curve-lending-js/pull/34