The DeploymentManagerImpl should pass in the IdentityManager to custom AuthenticationMechanismFactory in deployment

While io.undertow.servlet.core.DeploymentManagerImpl passes the deployment IdentityManager to the well known auth mechanisms, BASIC, FORM, ... it is not part of the io.undertow.security.api.AuthenticationMechanismFactory interface contract.

This leaves accessing the IdentityManager from the io.undertow.security.api.SecurityContext#getIdentityManager() method which is deprecated.

Since it looks like Java 8 is required, this could easily be updated via a new default method in AuthenticationMechanismFactory. I'll create a pull request for an example solution for review.