Contract handling helper

[fryorcraken]

Problem

Retrieving membership details and adding them to the RlnInstance is something that any user of js-rln should do and are likely to copy-paste from https://github.com/waku-org/js-waku-examples/blob/b6d9942b113e51298ba9e4405997c3dc52bec908/rln-js/index.html#L259

Same re listening to contract event add new memberships to RlnInstance.

Same re registering membership in contract: https://github.com/waku-org/js-waku-examples/blob/b6d9942b113e51298ba9e4405997c3dc52bec908/rln-js/index.html#L287

Solution

Provide a function helper that does that.

2 possibilities:

1. Function helper uses ethers, hence we can set ethers as an optional dependency
2. Function helper gets passed an object that implement an interface so to give the choice to the user between ethers and web3js.

Not sure (2) is straight forward, might be easier to just go with (1) for now

Acceptance Criteria

• [x] User can call 1 function to retrieve members from contract, add them to RlnInstance and add new members to RlnInstance by listening to contract
• [x] User can call 1 function to add credentials to contract.

[weboko]

I am still not able to determine good API to cover this so @fryorcraken do you see other points for improvement here?

[fryorcraken]

I see you have done improvement already with #43 . Let's use it in the examples and see how it looks.

[weboko]

Ok, so I'll define as follow up item next thing:

• [x] use contract abstraction in the example;