Support for Client ID Documents

[michielbdejong]

As per https://github.com/solid/solid-oidc/blob/a5a966c7342da01a57bfb316e5533ea7d82fd245/index.bs#L163-L166

The serialized JSON form of a Client ID Document SHOULD use the normative JSON-LD @context provided at https://www.w3.org/ns/solid/oidc-context.jsonld such that the resulting document produces a JSON serialization of an OIDC client registration, per the definition of client registration metadata from [!RFC7591] section 2.

[michielbdejong]

This also implies that clientId is an IRI which means it doesn't easily fit into routes like https://github.com/pdsinterop/solid-nextcloud/blob/7162a2c30be90aeb9c1d636eae19f60bcfc751d7/solid/appinfo/routes.php#L13-L15