Provide Lambda context in event object

[alephyud]

Fixes #856.

Adds a context attribute for the event argument of Cloudwatch, S3, SNS and SQS Lambda handlers, as suggested in the issue discussion. That can be used for logging or other purposes:

@app.on_sns_message(topic='my-demo-topic')
def handle_sns_message(event):
    app.log.debug("Received message with subject: %s, message: %s, request id: ",
                  event.subject, event.message, event.context.aws_request_id)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #1044 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1044      +/-   ##
==========================================
+ Coverage   95.47%   95.47%   +<.01%     
==========================================
  Files          27       27              
  Lines        4488     4489       +1     
  Branches      563      563              
==========================================
+ Hits         4285     4286       +1     
  Misses        131      131              
  Partials       72       72

Impacted Files	Coverage Δ
chalice/app.py	96.77% <100%> (ø)	⬆️

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[jamesls]

Thanks for the pull request, taking a look now.

[jamesls]

Looks good to me, let's get one more person to review this.

cc @kyleknap @stealthycoin

[jamesls]

I realize we have to do this because SQSRecord subclasses BaseLambdaEvent, but I wonder if we need this attribute on child events. You can always access it from the parent event.context attribute.

I suppose there's not really any downsides to passing this through all the child events that are created. We'll also have to do this when we implement dynamodbstream/kinesis events, which have a similar pattern.

cc @kyleknap @stealthycoin

[stealthycoin]

Looks good to me.