[Filebeat] AWS cloudtrail module should handle Digest files

[leehinman]

Describe the enhancement:

AWS Cloudtrail can be configured to create a digest of each cloudtrail
log file when it writes it. Filebeat should be able to ingest these
files.

https://docs.aws.amazon.com/awscloudtrail/latest/userguide/cloudtrail-log-file-validation-intro.html

[elasticmachine]

Pinging @elastic/integrations-platforms (Team:Platforms)

[elasticmachine]

Pinging @elastic/siem (Team:SIEM)