fix(ios): Support CORS when using Live Reload

[diachedelic]

_capacitor_file_ HTTP requests fail CORS checks when server.url is set. This PR implements CORS support on iOS. (Sidenote: does not require a fix for Android as _capacitor_file_ requests are sent to the same origin of the page).

How to reproduce

// capacitor.config.json
{
  "server": { "url": "http://192.168.15.26:6789" }
  ...
}

// my-app.js
const url = Capacitor.convertFileSrc(uri)
await fetch(url) // throws

[Error] Origin http://192.168.15.26:6789 is not allowed by Access-Control-Allow-Origin.
[Error] Failed to load resource: Origin http://192.168.15.26:6789 is not allowed by Access-Control-Allow-Origin. (1588911030406.txt, line 0)
[Error] Fetch API cannot load capacitor://localhost/_capacitor_file_/Users/me/Library/Developer/CoreSimulator/Devices/E60456F7-630E-4574-B7BD-A486C85B06C3/data/Containers/Data/Application/FFE5A297-770F-418D-9331-5833B1CCA94C/Documents/1588911030406.txt due to access control checks.
[Error] TypeError: Origin http://192.168.15.26:6789 is not allowed by Access-Control-Allow-Origin.
  capacitorConsole (user-script:2:78)
  (anonymous function) (bundle.js:4179)
  promiseReactionJob

[imhoffd]

@diachedelic Thanks for the PR! We are going to address this in Capacitor 3 using something like this: #3180

[realityfilter]

As this is not addressed in Capacitor 3. Can this pull request be merged to be aligned with the android implementation?

[diachedelic]

@realityfilter PR #3433 supersedes this one.