Retrieve JSON Web Key Set (JWKS)

Fetches the public keys that can be used to verify JWTs signed by Prolific. Clients should cache these keys and update them at least daily. To verify the signature of a JWT you must verify the following: * The JWT signature is authentic by verifying it with the public key from Prolific that correlates with the KID. * The JWT hasn't expired, by checking the `exp` claim. * The `aud` claim is the correct domain for your tool. * The `prolific` value matches your expected payload as set in the `external_study_url` property.