OIDC: Attempt dynamic registration with OP

[kerryarchibald]

Client implementation guide

Attempt dynamic client registration with OP

Register client with the OP using

{
            client_name: <SDKConfig brand>,
            client_uri: <absolute app url>,
            tos_uri: "https://element.io/terms-of-service",
            policy_uri: "https://element.io/privacy",
            response_types: ["code"],
            grant_types: ["authorization_code", "refresh_token"],
            redirect_uris: <absolute app url>, // not used yet
            id_token_signed_response_alg: "RS256",
            token_endpoint_auth_method: "none",
            post_logout_redirect_uris: <absolute app url>, // not used yet
        };

TODO:

• tos and privacy policy links are already in various parts of config.json, but not positively identifiable by key. Add to oidc specific config? add elsewhere on config?

{
    "privacyPolicyUrl": "https://element.io/privacy",
     "termsOfServiceUrl": "https://element.io/terms-of-service",
}

hydrogen implementation PR
MSC2966
RFC7591

AC:

• Where EW config.json doesn't have TOS and policy URLs: log error, exit flow and fall back to legacy login
• When registration fails, exit flow and fall back to legacy login
• When registration succeeds, continue to attempt OIDC auth
• Store response client_id for use in further interactions with OP