Implement custodial service

[sisyphusSmiling]

Problem

Currently, keys are stored locally client-side. This is not very secure and causes problems submitting multiple transactions (#85). Issues also arise because app-managed accounts need a reference point to determine the associated address of the app-managed account which, up until this point, have been handled with an onchain resource AccountCreator that goes against best practice and isn't scalable.

Suggested Solution

Since this repo is intended to be a reference for builders, we should implement at least one custodial service provider for account management. This would better represent best practices and showcase hybrid custody implementation end-to-end.