Currently there are at least three types of it that a Credentials implementation can have its config.jelly invoked with:

• CredentialsStoreAction.DomainWrapper when creating a new credential from the credential domain's screen
• CredentialsStoreAction.CredentialsWrapper when updating an existing credential
• CredentialsSelectHelper.WrappedCredentialsStore when performing an in-place Add from a job page.

The last one is particularly problematic as there is no guaranteed way to get the backing context object from that.

We should standardize on a single object type... it would seem that the CredentialsStore would be the ideal candidate.... leaving instance to be the existing Credential in the event of an update.