How to refactor METAccount so we can pass in our own service and access group to keychain?

[cerupcat]

Right now METAccount uses the bundleID and no access group for storing account credentials. We're writing an extension and we'd like to be able to share the credentials across extensions (and possibly other apps that have same credentials). In order to do this we need to have control over setting the serviceName and access group on METAccount. This required a good amount of refactoring since METAccount isn't currently accessible from METDDPClient. Any recommendations on how to possibly accomplish this that would be extensible for others (rather than us just hardcode in the values).