Add support for Elliptic Curve JWT algorithms

[atanunq]

This small patch checks if the algorithm starts with ES and if it does, correctly sets the key value. Fixes #442

Added a test that:

• generates a public/private key pair
• makes the public key accessible through the JWKS endpoint
• generates a signed JWT with the private key
• verifies the signature using the public key

Decided to go for a more complex test case because the django-oidc-provider library referenced in the local development guide does not support ES algorithms. Please let me know if you this that is not okay.

Also tested end-to-end with a locally hosted IdP that has support setting ES256 as the signing algorithm - Authentik

[akatsoulas]

Closing in favor of #513. It's the same PR rebased against main and squashed into a single commit.