Wallet integration into waku-rln-relay

[staheri14]

Problem

waku-rln-relay requires interaction with a smart contract in two phases: Registration and Slashing For both, the user needs to sign the transaction to deposit/Withdraw some Ether. Sending transactions requires access to the user's Ethereum account private key. It would not be secure to ask the user's private key directly to the waku-node. Thus, we need to use an external wallet app to perform the transactions upon request from the waku node.
This issue is to explore existing options for wallet integration into nim-waku and to identify the most convenient way to do so.

Potenitial solution

Can look into how this is done in nimbus.

[staheri14]

cc: @oskarth

[oskarth]

cc @jm-clius as well

[jm-clius]

Done and tracked elsewhere.