Remove DigestAuthenticationMechanism code duplication between Wildfly, Undertow and Elytron

Digest authentication mechanism is currently implemented in 3 places - in Wildfly undertow subsystem (legacy web authentication), Undertow itself (web console authentication) and Elytron (web authentication).

Any issue found in one of those scenarios is likely to affect others requiring fixing multiple codebases - ideally there should be a single implementation.