feat(server): automatically fund manager and staff wallets

[mfw78]

Problem
The manager and staff wallets are handled autonomously by the server, slowly consuming gas. Eventually they will need to have gas topped up, which creates an administrative burden.

Solution
Create a dripper contract that is authorised with the finance role on the respective service provider. When balances get low on the manager and/or staff accounts, they can automatically call a drip() method on the contract, allowing them to be topped up with gas.

Other solutions considered
TBD

Context
TBD