Add AWS SQS and SNS inputs

## Description
Add support for reading messages directly from SQS and SNS. We indirectly support this wit the the CloudTrail input, but we have received at least one request from a customer asking to be able to read messages from an SQS queue directly.

## References

See https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-how-it-works.html
https://docs.aws.amazon.com/sdk-for-java/v2/developer-guide/welcome.html

Also see the indirect code that reads from an SQS queue for the CloudTrail input:
https://github.com/Graylog2/graylog-plugin-aws/blob/dc36a579fb047772e1c14a46c4144f10b4f3521b/src/main/java/org/graylog/aws/inputs/cloudtrail/notifications/CloudtrailSQSClient.java#L40