[AWS] Add `custom` variable to all Cloudwatch inputs and at the package level

[kpollich]

In #9865, we ported the custom variable exposed in the aws_log integration here to all aws-s3 inputs in the AWS integration. We should also expose this variable on all aws-cloudwatch inputs, and we should add a top-level custom block to the integration as well to support multiple levels of customization + overrides for power users.

[sheikharsalanelastic]

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[botelastic]

Hi! We just realized that we haven't looked into this issue in a while. We're sorry! We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1. Thank you for your contribution!