Add authentication mechanism for DIGEST

Jakarta Security 2.0 provides support for the authentication mechanisms that were originally defined in Servlet, namely FORM and BASIC. Servlet defines two additional mechanisms: DIGEST and CLIENT-CERT. These had not been implemented due to time constraints.

For completeness DIGEST should be added to Jakarta Security as well.

Start by reviewing how the existing FORM and BASIC mechanisms are implemented in Jakarta Security and compare their integration points with the Servlet DIGEST definition. Determine the relevant entry points and tests, then confirm that DIGEST authentication is supported consistently with the existing mechanisms.

Written by the indexing model from the issue text.