Support ingestion from Custom CloudWatch Logs

[JoeShi]

Discussed in #178

^{Originally posted by teemuniiranen August 3, 2023}
I don't see CloudWatch Logs in the Service Log options but I undestand that it's used in some pipeline configurations. Can you create pure Log Group subscriptions with Centralized Logging or do I need to handle these manually from member accounts with "Create Amazon OpenSearch Service subscription filter" functionality?

[JoeShi]

There is a feature allowing customer to query CWL from OpenSearch, the issue is no longer valid.