Define CloudWatch Alarm Payload structure

[paritosh16]

Issue

Tries to solve an issue similar to #51 for CloudWatch Alarm Payload in a SNSEvent. If this this merged, can do another PR to resolve #51

Description of changes

TL;DR

Added struct for CloudWatch Alarm payload.

Details

• CloudWatch Alarm information lands up in SNSEntity.
• This payload is a valid json string in the SNSEntity.Message containing information about the alarm that changed state and published to SNS topic triggering the lambda function.
• CloudWatch Alarm payload can contain either:
  • Metrics: A list inside the Trigger section of the payload.
    • Metrics contains the list of metrics alarm is based off.
  • Metric: Details of the single metric that the alarm is based off.
• A sample event containing Metrics can be found here.
• A sample event containing single Metric information can be found here.

Structs Details

• The CloudWatchAlarmSnsPayload struct refers to first level in the JSON containing information.
• The CloudWatchAlarmTrigger struct refers to the trigger information.
• The CloudWatchMetricDataQuery struct refers to the list of Metrics inside Trigger.
• The CloudWatchMetricStat struct refers to the MetricStat key inside the Metric Data Query.
• The CloudWatchMetric struct refers to the Metric information inside MetricStat.
• The CloudWatchDimension struct refers to Dimensions of the metric inside CloudWatchMetric.

Why so many structs?

• All the structs defined except CloudWatchAlarmSnsPayload are already defined in the CloudWatch module in the AWS GO SDK.
• However, the structs in the CloudWatch module don't have any json definitions and hence to be in sync with how SNS related structs are defined, these have to defined again.
• Finally, all structs can be defined as nested ones in a single CloudWatchAlarmSnsPaylaod but for better readability nested structs are defined separately.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Merging #296 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #296   +/-   ##
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  Coverage   74.06%   74.06%           
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  Files          18       18           
  Lines         833      833           
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  Hits          617      617           
  Misses        152      152           
  Partials       64       64           

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[bmoffatt]

Thanks!