ExpressReceiver support for rawBody for signature verification

[aoberoi]

Description

The ExpressReceiver handles buffering the request body stream, and then verifying the signature (using the signing secret) in order to authenticate the sender as Slack.

However, when running in some serverless environments, such as Google Cloud Platform (GCP), the request body has already been buffered and even parsed before the Receiver gets a chance to interact with it. In GCP, the original buffer is not destroyed, but rather added as the req.rawBody property.

The ExpressReceiver should check the req.rawBody property for a buffered body for verification, instead of failing in this condition.

This feature request is analogous to slackapi/node-slack-events-api#85, and the fix would be analogous to slackapi/node-slack-events-api#90.

Requirements (place an x in each of the [ ])

• I've read and understood the Contributing guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've searched for any related issues and avoided creating a duplicate issue.

[aoberoi]

cc @seratch - I think you'd like to follow this issue 😄

[seratch]

Did somebody already start working on this? If not, I'll make a pull request soon (possibly this weekend or early next week).

[aoberoi]

@seratch nobody has claimed this one yet that we're aware of. the timeline of next week sounds great! i'll assign this one to you just to keep track 🙇