The foundational traits and types for the huskarl OAuth2 ecosystem.
Most applications depend on the higher-level huskarl crate (grants, token
cache, authorizer) rather than this crate directly. huskarl-core is the shared
base they build on: the one concrete Error/ErrorKind, the
HttpClient abstraction, client authentication
(client_auth), DPoP (dpop), the JOSE primitives (jwt, jwk,
crypto), secret handling (secrets), and authorization-server metadata
(server_metadata). Implement its traits to plug in a transport, crypto
backend, or secret source.