Add authentication-failure-handler-ref to <session-management>

[wonderfulrosemari]

Closes gh-2000

This PR adds XML namespace support for configuring a custom AuthenticationFailureHandler on <session-management>.

It introduces support for:

<session-management authentication-failure-handler-ref="..."/>

Changes include:

• parsing and wiring authentication-failure-handler-ref in HttpConfigurationBuilder
• validation to prevent using authentication-failure-handler-ref together with session-authentication-error-url
• schema updates in namespace RNC/XSD files (5.8 through 7.1)
• session-management namespace appendix documentation update
• integration tests for custom failure handler invocation
• integration tests for mutual-exclusion parsing error when both attributes are configured

[rwinch]

Thank you for your contribution @wonderfulrosemari

Is this causing you any particular issues? I ask because, while not deprecated (we still fix bugs), generally XML Namespace support is in maintenance mode (no new features) in favor of Java Configuration.

[rwinch]

Thank you again for the submission. After discussing this with the team, I'm going to close this because XML Namespace support is effectively in maintenance mode. This means that while we will fix bugs, we do not expect to enhance it going forward.