[OIDC] Make API Authorization(resource) optional

[nakamume]

The OIDC plugin sends resource request for api authorization. The request is sent to https://graph.microsoft.com by default.

We use OIDC plugin just for authentication and not for any API authorization. In current state our IdP integration will always fail with Error in OpenID Connect: unauthorized ‘resource’ requested

Could we make Resource param null by default.

I'll open a PR for this

Edit: Opened a PR https://github.com/microsoft/moodle-auth_oidc/pull/48