out_cloudwatch: add entity support and remove unnecessary log content

[zhihonl]

Background

CloudWatch introduced a new feature called Explored Related which is a UI component that helps user navigate between their telemetry(metrics,logs) and AWS resources. This is done by including a new field in PutLogEvents API which is called Entity. More documentation about this new field can be found here: https://docs.aws.amazon.com/AmazonCloudWatchLogs/latest/APIReference/API_Entity.html

Problem

CloudWatch output plugin is what sends user logs to the backend. It needs to populate the entity field in the PutLogEvents field. As part of this effort, we need to add additional logics in CloudWatch plugin to attach appropriate entity data then make the correct PutLogEvents call.

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Testing
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• Example configuration file for the change
• Debug log output from testing the change

• Attached Valgrind output that shows no leaks or memory corruption was found

If this is a change to packaging of containers or native binaries then please confirm it works for all targets.

• Run local packaging test showing all targets (including any new ones) build.
• Set ok-package-test label to test for all targets (requires maintainer to do).

Documentation

• Documentation required for this feature

Backporting

• Backport to latest stable release.

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