Add alg to SigningKey types (#220)

Co-authored-by: David Patrick <david.patrick@auth0.com>

Author: okko

index.d.ts

@@ -44,6 +44,7 @@ declare namespace JwksRsa {
 
   interface CertSigningKey {
     kid: string;
+    alg: string;
     getPublicKey(): string;
     publicKey: string;
   }
@@ -63,6 +64,7 @@ declare namespace JwksRsa {
 
   interface RsaSigningKey {
     kid: string;
+    alg: string;
     getPublicKey(): string;
     rsaPublicKey: string;
   }