Add clientOptions.logger option

[aoberoi]

Description

We'd like to support the use case of customizing the logger for the WebClient instances that the Bolt App creates. Some of the motivation and ideas around this change were discussed and agreed upon here: #278 (comment)

Side note: in the end we'd like to accomplish the following

• logLevel affects the App instance but also cascades to all owned objects with loggers.
• to customize the Receiver’s logger, you’re expected to set the receiver option to a fully initialized receiver with your custom logger already set.
• to customize the WebClient’s logger, you’re expected to set the clientOptions.logger option (that's what this issue is about).
• to customize the App’s logger, use the logger option.

We should probably have tests that verify all the cases above ☝️

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]

This is still in progress but it's a pretty easy task.

• https://github.com/slackapi/node-slack-sdk/blob/%40slack/web-api%406.1.0/packages/web-api/src/WebClient.ts#L495
• https://github.com/slackapi/bolt-js/blob/%40slack/bolt%403.3.0/src/App.ts#L240-L246