Implement private_key_jwks client authentication

This involves a lot of things, including:
 - better VerifyingKeystore trait
 - better errors in the JOSE crate
 - getting rid of async_trait in some JOSE traits

crates/jose/src/lib.rs

@@ -26,7 +26,7 @@ pub use self::{
     jwk::{JsonWebKey, JsonWebKeySet},
     jwt::{DecodedJsonWebToken, JsonWebTokenParts, JwtHeader},
     keystore::{
-        ExportJwks, JwksStore, SharedSecret, SigningKeystore, StaticJwksStore, StaticKeystore,
+        DynamicJwksStore, SharedSecret, SigningKeystore, StaticJwksStore, StaticKeystore,
         VerifyingKeystore,
     },
 };