Add client to the list of args send through to listeners

[marks]

Description

Similar to #168, I'd like to propose that listeners (such as event, message, etc.) be able to access the @slack/web-api client. This would make it easier for listeners to make calls to the Web API methods while keeping the client rate-limit and queue-aware.

Without this, I either need to initialize my own web client in each file with listeners or pass app.client around manually

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.

[aoberoi]

I think this is a good idea. There's no reason that I can think of that app.client is not offered to the middleware and listeners in an application.

[seratch]

I agree this is a missing piece for common use cases.

Regarding the possible changes, are you thinking to add client and logger to listener args like https://github.com/slackapi/bolt/blob/%40slack/bolt%401.5.0/src/types/command/index.ts#L7-L14 or add them to context? I prefer the former.

[marks]

Hey @seratch - definitely want to add client and logger as listener args, not context