Allow override of response_url on api wrapper methods

[Ambro17]

Description

I want to use the pythonic api like Slack.chat_postMessage(...) when responding async to a command. As we know slack requires responses to be sent within the first 3 seconds.

If we take too long, a response_url is given to send responses there for up to 30 minutes.

Currently, the wrapper methods all fall into self.api_call base method that in its definition gets the api_url from

api_url = self._get_url(api_method)

Source code link:

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.

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• slackclient version: 2.1.0
• python version: 3.6
• OS version(s): ubuntu 18

[seratch]

related to: #270

[seratch]

As we'll be tracking this feature request with #270, allow me to close this issue now. Thank you very much for taking the time to share your feedback.