Add api_app_id in Slash command payloads

[stevengill]

Description

Slash command invocations will now include an api_app_id parameter with your Slack app's ID. This parameter will better assist you in handling commands from multiple applications or environments.

https://api.slack.com/interactivity/slash-commands
https://api.slack.com/changelog

What type of issue is this? (place an x in one of the [ ])

• bug
• enhancement (feature request)
• question
• documentation related
• testing related
• discussion

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.