[RFC] support authentication challenge callback via plugin

[brodycj]

Platforms affected

iOS

Motivation and Context

I think it would be nice if we could support plugins that can handle an authentication challenge coming from the server, namely this callback on WKWebView:

• https://developer.apple.com/documentation/webkit/wknavigationdelegate/1455638-webview?language=objc

It would help support this functionality in a plugin that I maintain:

• iOS WKWebView support needed cordova-ccafix/cordova-plugin-client-certificate-support#7

Description

This proposal adds a handler for a "didReceiveAuthenticationChallenge" callback from the WKWebView, and hands it off to a plugin if possible.

I am raising this proposal now to see whether or not there is sufficient interest to justify any further work. A couple major TODO items:

• cleanup logging
• cleanup some comments
• find a better completion strategy than "first plugin wins"

Alternative solution: leave this functionality up to a native XHR plugin to support, which is good enough for my customer need right now

Testing

Tested with this proposal on my plugin:

• expose authentication challenge callback to support iOS WKWebView cordova-ccafix/cordova-plugin-client-certificate-support#12

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

Merging #1212 (c194fc0) into master (67b0bb2) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1212   +/-   ##
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  Coverage   78.53%   78.53%           
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  Files          15       15           
  Lines        1761     1761           
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  Hits         1383     1383           
  Misses        378      378           

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