[image_picker] optionally disable PHAsset & iOS permission request

[cpboyd]

Description

Make iOS 11+ permissions requests optional per https://developer.apple.com/forums/thread/653414.
PHAssets aren't actually required and there was already some fallback code in place.

To keep this as a non-breaking change, I added an optional parameter of iosPhaAsset with a default of true.
Setting this to false will disable any permissions checking on iOS and simply display the image picker.

I still need to update the tests.

Related Issues

Fixes flutter/flutter#65995

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[cpboyd]

This may actually need a check for iOS 11 or higher

[cpboyd]

As above, this may need a check for iOS 11 or higher

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[cpboyd]

@cyanglaz Per flutter/flutter#65995 (comment), should I change iosPhaAsset to forceFullMetaData or are you working on your own PR?

[cyanglaz]

Thanks for sending out this PR! I had a comment about this new flag here flutter/flutter#65995 (comment)
Could you rename the parameter as I commented?

The other changes would be pretty similar.

Integration testing this might be a little tricky. I will work on a PR to add some tests for you. Meanwhile, could you add some unit tests for this change?

[cyanglaz]

@cpboyd I haven't started working on this yet, I'd like to help you land this PR :)
I have landed a PR to set up some XCUITests for the plugin, it will help you add integration tests in your PR. #3254

Please feel free to ping me if you have any questions, I'd be happy to review once the tests are added.

[cyanglaz]

@cyanglaz Per flutter/flutter#65995 (comment), should I change iosPhaAsset to forceFullMetaData or are you working on your own PR?

Oh, I didn't see this :) yes, please rename it to forceFullMetaData, and add dart doc on the methods with the same message I added in my comment.

[cpboyd]

@cyanglaz It seems that since I modified my fork to be based off of the main repo (rather than another fork), it messed with GitHub's PR system.

I'll close this and re-open with the new changes.