fix: CMI and Vault registration issue - #576

The Vault economy integration was resolved once, synchronously, during
plugin onEnable. If the actual economy provider (e.g. CMI) registers
with Vault's ServicesManager after that point - which can happen since
plugin enable order between two plugins that both merely soft-depend
on Vault isn't guaranteed - the lookup returned null and no economy
integration was ever registered. EconomyManager then failed silently
on every balance operation for the rest of the server's uptime.

Add a ServiceRegisterEvent/ServiceUnregisterEvent listener that keeps
the registered Vault integration in sync with whatever Economy
provider Vault currently has, instead of relying on a single lookup
at boot.