CloudTrail validate logs backfill digest support (aws#10058)

* Add backfill digest file validation for CloudTrail validate-logs

* merge branch develop to cloudtrail-validate-logs-backfill

* Add a change log entry to the cloudtrail validate logs backfill support

---------

Co-authored-by: Ian Lee <ianleeyt@amazon.com>

Author: IanLeeYT

.changes/next-release/enhancement-cloudtrail-61479.json

@@ -0,0 +1,5 @@
+{
+  "type": "enhancement",
+  "category": "cloudtrail",
+  "description": "Added support for validating backfill digest files in ``validate-logs`` command"
+}