LibCrypto: Implement HMAC

Author: https://github.com/alimpfard
Commit: f2cd004d11
Pull-request: https://github.com/SerenityOS/serenity/pull/1661
Reviewed-by: https://github.com/Dexesttp
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/itamar8910

Libraries/LibCrypto/Cipher/Cipher.h

@@ -129,6 +129,8 @@ namespace Cipher {
         virtual void encrypt_block(const BlockType& in, BlockType& out) = 0;
         virtual void decrypt_block(const BlockType& in, BlockType& out) = 0;
 
+        virtual String class_name() const = 0;
+
     private:
         PaddingMode m_padding_mode;
     };