Support OIDC Discovery

[spuxx-dev]

Fully-fledged OIDC clients commonly utilize OIDC Discovery, which lets them automatically configure their communication with the upstream IDP.

While potber-auth only supports a very small part of the OAuth spec, it might actually be possible to satisfy proper OIDC clients by including a .well-known metadata file. This would allow for a wider range of applications to be used with potber-auth.

Links:

• https://auth0.com/docs/get-started/applications/configure-applications-with-oidc-discovery#sample-response
• https://openid.net/specs/openid-connect-discovery-1_0-final.html#RFC5785