SAML Metadata support

[twenzel]

When moving from RSK.Saml to Duende the Metadata feature is missing.
It should be possible to define a metadata url from the SAML IdentityProvider on the SAML ServiceProvider. This metadata url should be used to retrieve the IDP metadata (SSO urls, certificates, etc). This metadata should be refreshed every X days.

[maartenba]

Thanks! This one is being tracked.