Add customer defined Cloudwatch Destination for subscription

[richardkeit]

Is your feature request related to a problem? Please describe.
Centralising logs across an organization requires a lot of effort (ie creating subscription filter per log group per account).
To create this, it typically requires creation an event driven architecture to create subscription once log group is created and then all the work required for managing it, updating runtime versions etc etc

Reason why this is required Elastic.io for the S3 integration requires the objects to be uncompressed

Describe the feature you'd like
Similar to the replaceLogDestinationArn to remove logGroup subscriptions (side note: I found this key name misleading I thought it would replace the LZA defined subscription with the one I defined, not remove the subscription defined by this key), would like the ability to add a log group subscription, allowing us to create a Kinesis Data Stream and Firehose outside of the LZA codebase.

Rationale to create it outside of the LZA is that we don't want to create a tightly coupled Firehose to the Data Stream that can change at any time.

Additional context
Even if you don't agree with the third party integration style, I think it can be agreed that maintaining logGroup subscriptions is a bit painful and this would help with that plumbing (as most the code is there already)