Split out hook service's business logic

When migrating the operation service into Go code, we'll need to manually call out to Git hooks. This is currently hard to do as there is no way to call into the hook service directly and reaching out to it via our own socket just doesn't feel right.

To make this easier, this MR splits the business logic out of the service into a separate hook manager, following the same pattern as we have it for Praefect. Like this, the operation service can directly call into the hook manager without having to do an RPC call.