feat(server): add JWT-based authorization mode

[kubkon]

Closes #812

This PR implements the proposal issue for JSON Web Token based authorization in the notary server. I tried my best to keep the addition of JWT validation mechanism the least invasive and thus the extraction logic is handled in existing AuthorizationMiddleware struct where we guide the extractor which header and token/API key to look for based on the server's config.

Server's config is now expecting an enum

enum AuthorizationModeProperties {
    Jwt(JwtAuthorizationProperties),
    Whitelist(String),
}

This way, only one mode is possible and this is enforced at config parsing level.

I have also added an integration test which mirrors that for the whitelist authorization but where we use JWT token for authorization.

Finally, I've updated the README and openapi spec to reflect the proposed changes.

Lemme know what you think! Hopefully this PR will make reading and deciding on the proposal in #812 a little easier since it outlines the number of required changes to make it a reality.