expose identity traits (#469)

* expose identity traits

* address feedback

Co-authored-by: Brian Caswell <bcaswell@microsoft.com>

Author: bmc-msft

sdk/identity/src/lib.rs

@@ -53,3 +53,4 @@ pub use errors::Error;
 pub mod refresh_token;
 pub mod token_credentials;
 mod traits;
+pub use traits::{BearerToken, ExtExpiresIn, RefreshToken};