Werk #14976: Add SAML authentication to Checkmk UI

SAML authentication is now integrated with the Checkmk UI.

The initial feature set includes the following:

• Single sign-on (HTTP POST binding/front channel communication)
• Setup page to configure one or more SAML connections: Setup -> Users -> SAML authentication
• Automatic user creation and user attribute synchronization at login time
• Signing of requests and signature verification of responses. Supported algorithms: SHA256, SHA384, SHA512
• Logging to $OMD_ROOT/var/log/web.log for administrative and debugging purposes
• Option to log in with username and password for non-SAML users (htpasswd/LDAP)

With this change, we also deprecate the previous SAML integration approach on Apache level based on mod_auth_mellon. Support will be dropped with Checkmk version 2.3.0. If you would still like to use this approach in version 2.3.0 and beyond, mod_auth_mellon will need to be installed.

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