Improved handling of unnecessary Cloudtrail messages

While setting up the Cloudtrail input, customer noted the following ERROR in their server.log:

`2022-10-27T13:27:38.655-04:00 ERROR [CloudtrailSNSNotificationParser] Parsing exception.
com.fasterxml.jackson.core.JsonParseException: Unrecognized token 'CloudTrail': was expecting ('true', 'false' or 'null')
at [Source: (String)"CloudTrail validation message."; line: 1, column: 11]
at com.fasterxml.jackson.core.JsonParser._constructError([JsonParser.java:1804](http://jsonparser.java:1804/)) ~[graylog.jar:?]`

Per https://github.com/Graylog2/graylog-plugin-aws/issues/80 the Cloudtrail input will skip processing/continue executing past messages it does not understand, and per https://github.com/Graylog2/graylog-plugin-aws/issues/249 we acknowledge that this is an unnecessary message.

However, this negatively impacts the customer experience by adding ERROR lines to their server.log with a scary-looking stack dump, and if customer is attempting to set up the Cloudtrail input, will be mindful of log lines that we specify to be ERROR.

A more ideal situation would be to parse the message type, and present something like "Cloudtrail validation message encountered, skipping processing for unnecessary message." at reduced log priority than ERROR.

[HS-1155337333](https://app.hubspot.com/contacts/8267556/ticket/1155337333)