Create a subgraph that tracks `StealthKeyChanged` events from `StealthKeyRegistry` contract

[garyghayrat]

Instead of usingqueryFilter (i.e. provider.getLogs()) to getBlockNumberUserRegistered in umbra-js/src/utils/utils.ts, we can use a subgraph to increase reliability.

[jferas]

I believe the use of the subgraph can also be extended to be used in lookupRecipient in umbra-js/src/utils/utils.ts as that is what is called on the original launch (or on chain switch) by the front-end to ascertain if the user has previously registered and thus triggers the display (or not) of the "Setup" button.