refactor: simplify OpenPaymentsService

[sidvishnoi]

OpenPaymentsService has grown a lot (1000+ lines) as we kept adding new things to this service directly. For a cleanup, it might be worth separating the connect wallet parts to a separate service. I think OutgoingPaymentGrantService might be a good name, as we use same grant part in connectWallet, addFunds, updateBudget, disconnectWallet and more (open to any better names).

We likely want to keep the token within this OutgoingPaymentGrantService. Might need some care with cyclic dependencies with awilix though.