[image_picker] Migrate image_picker to package:cross_file

[BeMacized]

Changed all usages of PickedFile to XFile, to follow the changes made to the platform interface (#4072)

This PR will remain in draft state until the platform interface changes (#4072), as well as the web implementation of it (#4083) have been merged and published, so that the pubspec can be updated properly.

Relevant issue:

• [image_picker] Migrate image_picker to package:cross_file flutter#70886

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

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[ditman]

image_picker_for_web will need to be landed before the core plugin, so we can update the endorsed version and the platform_interface in one go.

Can you split the changes to image_picker_for_web to a new PR?

[BeMacized]

@ditman Certainly! The changes have been reverted and moved to a new PR: #4083

[BeMacized]

@ditman It should now be ready to go!

[ditman]

I think this is ready to go. I'm going to give a manual try to the example app in a couple of platforms (android, web) just to quickly verify.

[ditman]

(Looks good on Web and Android. Note that web has not implemented the "multi image picking" feature.

[ditman]

Thanks @BeMacized!!