Add client callback for API warning messages

[ghost]

Dupe of #905, except that it doesn't contain all the changes to our forked master branch anymore.

---

Problem: There's no way to surface Slack's API warnings in application code from the library.

This PR exposes a new client Option that registers a callback for every response that comes back with warning information. This suits the use case for logging/tracking warnings as they appear, without affecting the request/response interface of each endpoint. The nodejs sdk seems to take a similar approach.

onWarning := func(warning *slack.Warning, path string, values url.Values) {
  // Log the warning here.
}
client := slack.New(token, slack.OptionOnWarning(onWarning))

An easy way to test this and see warnings is to update outgoing headers to have ; charset=utf-8 at the end.

Relevant docs/reading:

• https://api.slack.com/web#slack-web-api__evaluating-responses
• https://api.slack.com/changelog/2020-01-deprecating-antecedents-to-the-conversations-api#warnings
• https://medium.com/slack-developer-blog/well-wishes-and-warnings-in-the-web-api-489456837220

[kanata2]

@ryannjohnson-range
Since #939 was recently merged, it is now possible for users to retrieve warnings.
If there is no special usecase where we want to execute the callback,
I think this PR should be closed, what do you think?

[ghost]

Warnings still look inaccessible if the response succeeds.

The use case I'm concerned about for warnings is logging, specifically to alert me when an API I'm using is being scheduled for deprecation. It makes sense to me to have this occur at the client-level since I don't expect my application logic to fork based on warnings.

[kanata2]

Warnings still look inaccessible if the response succeeds.

Ah,I missed this case. Thanks!