SNS Message Attributes should use the new AWS SDK types and support binary values

[matiu3]

https://github.com/LegNeato/aws-lambda-events/blob/632a850ed90503dbe07fceff1f354fc1df289c4c/aws_lambda_events/src/sns/mod.rs#L89

New SDK type: https://docs.rs/aws-sdk-sns/latest/aws_sdk_sns/model/struct.MessageAttributeValue.html

This way, they could support binary values.

(I discovered this because I'm making a lambda, that copies SNS messages cross account. Currently I have to manually convert.

Thank you for this great work.

[calavera]

that's nice. Do you want to open a PullRequest and add the changes?