feat(core): implement Identity, IdentityProvider trait, and ConfigIdentityProvider

Add Identity struct with id/scopes/resources fields and IdentityProvider
trait with resolve_from_fingerprint/resolve_from_token methods. Implement
ConfigIdentityProvider reading from ArcSwap<DynamicConfig.auth> for
fingerprint-based key lookups. Delegate ServerHandler::auth_publickey()
through IdentityProvider instead of direct AuthPolicy access. Store
authenticated Identity in the handler for use by ForwardingPolicy.

crates/alknet-core/src/lib.rs

@@ -61,6 +61,7 @@ pub mod transport;
 #[cfg(feature = "testutil")]
 pub mod testutil;
 
+pub use auth::{AuthToken, ConfigIdentityProvider, Identity, IdentityProvider};
 pub use client::channel_manager::{ChannelManager, ForwardRequest};
 pub use client::connect::{ClientSession, ConnectError, ConnectOptions, TransportMode};
 pub use config::{