Add support for SAML authentication

[ray-lee]

Is your feature request related to a problem? Please describe.

DSpace/DSpace#9438 adds support for SAML authentication to the back-end. This functionality needs to be exposed in the UI.

Describe the solution you'd like

Add a "Log in with SAML" option to the log in menu, when SAML authentication is enabled in the back end.

Describe alternatives or workarounds you've considered

Additional context

I'm working on a PR for this.

This will be essentially the same as other authentication methods, like OIDC, Shibboleth, and ORCID.