v0.4.0

• switch to Actix Web 4 (Actix Web 3 is no longer supported)
• handle extraction and parsing of custom JWT claims
• add a way to access parsed roles from handlers (every Keycloak role contained in the JWT)
• add compatibility with the paperclip crate (under the paperclip_compat feature)
• add passthrough policy setting to allow auth to be optional
• add a KeycloakAuth::default_with_pk() helper function to initialize the middleware with default settings
• improve extractors error types
• expose a pure function to extract custom JWT claims from an Actix Web request
• update to Rust 2021 edition