[UNDERTOW-34] Update to IdentityManager and DigestMechanism to use a DigestCredential

An update to the IdentityManager and DigestMechanism to make use of a DigestCredential for the verification of Digest requests.

The corresponding WildFly commit is here: -

https://github.com/darranl/wildfly/commit/25d5bd4733bae504eef4001baacb71409e4c7ca0

Also removed methods from the IdentityManager not required by Undertow, the purpose of this IDM package is as a SPI to identify the minimum requirement of an IdentityManager implementation to be able to operate with Undertow.  IdentityManager implementations with more complex functionality can be provided by the integration with WildFly, Undertow however should not be imposing a requirement on an implementation more complex than is required.