[NEXUS-5490] Introduce NexusAuthenticationFilter that delegates AuthenticationToken creation to available AuthenticationTokenFactories

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
[NEXUS-5490] Bind authcBasic/authcNxBasic to NexusAuthenticationFilter that will use factories first to create authentication tokens, falling back to current implement ation from NexusSecureHttpAuthenticationFilter

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge remote-tracking branch 'origin/master' into NEXUS-5490-remote-user
Merge remote-tracking branch 'origin/master' into NEXUS-5490-remote-user
Merge remote-tracking branch 'origin/master' into NEXUS-5490-remote-user
Merge branch 'refs/heads/master' into NEXUS-5490-remote-user
[NEXUS-5490] nexus-rutauth-plugin (add support for REMOTE_USER header)

Signed-off-by: Alin Dreghiciu <adreghiciu@gmail.com>
Merge remote-tracking branch 'origin/master' into NEXUS-5490-remote-user
Merge remote-tracking branch 'origin/master' into NEXUS-5490-remote-user
Merge pull request #112 from sonatype/NEXUS-5490-remote-user

REVIEW: [NEXUS-5490] Extensible authentication token factories & nexus-rutauth-plugin