Add better support for App Level Tokens

[stevengill]

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.

[seratch]

Not sure if there are any other web-api methods that currently do.

As you mentioned, apps.event.authorizations.list & apps.connections.open are the only ones that require an app-level token.

@stevengill
Do we want to add appToken in the next minor release(v6.2)? This sounds like a minor improvement to me but it may be helpful for some use cases.

[seratch]

We moved this task to v6.3 or later.