feat: allow the auth context to be configured

This is useful because ADFS does not recognize the default value in this plugin (`urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport`) and wants `http://schemas.microsoft.com/ws/2008/06/identity/authenticationmethod/windows` in there instead.

I believe you can also set `disableRequestedAuthnContext` to `true` to get this to work. Since this parameter was not documented I also added an entry for it.