Move as much functionality as possible into the adapter class

[aoberoi]

Description

try to bring token verification, error creation, and/or emit argument building into the adapter class. one day this may become shared among multiple web framework outlets.

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

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

Requirements

• 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.

[shaydewael]

slackapi/node-slack-sdk#815