getOidcIssuerUri
Returns the OIDC issuer URI advertised by Link: <...>; rel="http://www.w3.org/ns/solid/terms#oidcIssuer", or null if absent.
Spec: https://solidproject.org/TR/oidc — Solid-OIDC issuer discovery
Returns the OIDC issuer URI advertised by Link: <...>; rel="http://www.w3.org/ns/solid/terms#oidcIssuer", or null if absent.
Spec: https://solidproject.org/TR/oidc — Solid-OIDC issuer discovery