S3Bucket struct is missing `awsGeneratedTags` field.

The `S3Bucket` struct of the S3 event is missing the new `awsGeneratedTags` field:
https://github.com/aws/aws-lambda-go/blob/87e344b1d4de6b5378079a153603e22ec1c77aa1/events/s3.go#L49-L53

It seems it had been introduced recently https://docs.aws.amazon.com/AmazonS3/latest/userguide/notification-content-structure.html.

Start with the S3Bucket struct in events/s3.go at the referenced lines and compare its fields with the S3 event notification structure in the linked AWS documentation. Add support for the missing field and verify that the repository’s S3 event parsing tests still pass.

Written by the indexing model from the issue text.