Wrong configuration default for aws_message_type

## Description
While looking into adapting the AWS custom input UI to work on the Forwarder, I found that the default value set in the configuration API for `aws_message_type` is using one of the AWS regions instead of one of the values set in [`AWSMessageType`](https://github.com/Graylog2/graylog-plugin-integrations/blob/75c588e03628e3d28a09359236b4f514fd543ef3/src/main/java/org/graylog/integrations/aws/AWSMessageType.java). This is where the configuration option is set:

https://github.com/Graylog2/graylog-plugin-integrations/blob/75c588e03628e3d28a09359236b4f514fd543ef3/src/main/java/org/graylog/integrations/aws/codecs/AWSCodec.java#L109-L116

I don't think users creating inputs through the custom UI are impacted by this, though, since the UI sets it's own default for that configuration option, so this will only affect those creating inputs through the API.

## Environment

- Graylog Version: Graylog-4.1.0-SNAPSHOT (Integrations 4a765d1fb544636354b9b4cc00cdd223f2ec33ae)