Add message_attributes to Message object as per SQS Message struct

[ghost]

• Add message_attributes to Message object as per SQS Message struct
• Return message_attributes and md5_of_message_attributes as part of receive_message call with optional message_attribute_names parameter
• Return md5_of_message_attributes as part of send_message call

[felixbuenemann]

Thanks for picking this up! 👏