feat: Allow environment variable expansion in agent.json

[frankfarrell]

For instance, if there is an env variable AWS_REGION, allow the following config to be valid:

{
    "cloudwatch.emitMetrics": true,
    "kinesis.endpoint": "kinesis.${AWS_REGION}.amazonaws.com", 
    "firehose.endpoint": "firehose.${AWS_REGION}.amazonaws.com"
...etc
}

[gorkemmulayim]

I would like to work on this. Can you assign this to me? @chaochenq

[chaochenq]

That would be great, thanks @RootG, although your name doesn't show up in the assigning list, i don't know how to assign it to you.

[gorkemmulayim]

@chaochenq Hi, it has been a while since I have opened a pull request about this issue. I believe the pull request changes will resolve the issue. Can you please review it?