refactor: Define a protocol for scheme-handling plugins

[dpogue]

Platforms affected

iOS

Motivation and Context

Existing behaviour for plugins to opt-in to handling custom scheme resource loads was not documented and there were some unsafe assumptions around only a single plugin handling all requests.

Description

Add a CDVPluginSchemeHandler protocol to allow plugins to declare that they want to intercept WebKit resource loads.
We don't rely on the protocol itself being implemented by the plugins (we continue to check with -respondsToSelector:) but this allows us to avoid objc_msgSend and provides a way to document some of this plugin behaviour that is not otherwise explained.

Also, use a NSMapTable to associate plugin instances with the request being handled. Since we loop over the plugins, there's a chance that multiple plugins might want to handle different requests (which sounds like a logistical nightmare, but it's definitely possible). Now we keep a map of which plugin is handling each request so that we can make sure the right plugin gets called when the resource load is cancelled. NSMapTable uses weak associations, so when the task is done and released, the key and value will be dropped from the table automatically.

This should also resolve the unsafe plugin iteration issue that was mentioned in GH-1272 and GH-1030 by always iterating over an array of plugin objects that is a copy (due to calling -allValues).

Testing

Existing tests pass. Tested manually in simulator with custom scheme.

Checklist

• I've run the tests to see all new and existing tests pass

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.22%. Comparing base (458f5f3) to head (6ba4aca).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1479   +/-   ##
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  Coverage   80.22%   80.22%           
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  Files          16       16           
  Lines        1871     1871           
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  Hits         1501     1501           
  Misses        370      370           

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[erisu]

LGTM - Tested and confirmed that a plugin with defined overrideSchemeTask is still executed.