Add better support for App Level Tokens

Description

Related to #1132, We need to add better support for App Level Token in @slack/web-api.

I'm thinking we either pass in App Level Token in the token property (this works today), or we introduce a new appToken property. If we introduce a new appToken property, we could add code to make certain api calls use appToken over token (which is usually a bot token). apps.event.authorizations.list & apps.connections.open currently require an App Level Token. Not sure if there are any other web-api methods that currently do.

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.

Start with the @slack/web-api package and review related issue #1132, then inspect how token options are handled for apps.event.authorizations.list and apps.connections.open. Define whether App Level Tokens use the existing token property or a new appToken property, identify any other affected methods, and confirm the chosen behavior for each required API call.