[cross_file] Updates cross_file to a package separated federated plugin

[bparrishMines]

Updates cross_file to a package separated federated plugin. Keeping the package separation is still debatable.

Follows the general design outlined at https://flutter.dev/go/flutter-file-system

Implementation is split into

• cross_file_io: for use cases supportable by dart:io (all platforms except Web)
• cross_file_web: for Web
  • Uses Blobs and Blob URLs.
  • Supports package:web Files.
• cross_file_android: for Android Scoped Storage
  • Uses the high level abstraction of DocumentFile to interact with content uris.
  • Uses Kotlin.
• cross_file_darwin: for iOS and macOS App Sandbox
  • Extends cross_file_io implementations to add support for following methods:
  • Adds support to call URL.startAccessingSecurityScopedResource and URL.stopaccessingsecurityscopedresource.
  • Adds support to bookmark URLs following: https://developer.apple.com/documentation/Security/accessing-files-from-the-macos-app-sandbox#Persist-file-access-with-security-scoped-URL-bookmarks

Pubspec dependencies have been changed to use ones from git to have runnable example apps while in development and review. Specifically because file_selector uses the current version of cross_file and I didn't want to make this a file_selector + cross_file PR. Can update it to this if preferred.
Example

dependencies:
  cross_file:
    git:
      url: https://github.com/bparrishMines/packages
      ref: new_crossfile
      path: packages/cross_file/cross_file
  file_selector:
    git:
      url: https://github.com/bparrishMines/packages
      ref: new_file_selector
      path: packages/file_selector/file_selector

The cross_file example will run for all platforms except for linux and windows since they are simply using the dart:io implementation.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to 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, or I have commented below to indicate which version change exemption this PR falls under¹.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³