[file_selector] Add getDirectoryPaths method to the file_selector

[eugerossetto]

This PR adds the file_selector implementation for retrieving multiple directories paths from a select folder dialog.

This is a continuation of PR #6572, PR #6573 and PR #6575 so these changes will be ready to merge once the new version 2.3.0 in file_selector_platform_interface becomes available and the Linux and MacOS implementations are ready, before that, there's no point for this to be merged.

Issue:
Support for selection of multiple directories, through desktop's native open panel, in 'file_selector' package #74323

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. (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, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[stuartmorgan-g]

Update from triage: this is still waiting on the implemention PRs to be reviewed and landed.

[stuartmorgan-g]

Status update:

• Linux is landed
• macOS is waiting for final review: [file_selector] Add getDirectoryPaths implementation for macOS #7115
• There doesn't seem to be a Windows implementation PR? If that was waiting for the Dart rewrite, I think it would be better to do the small fix to implement this first to unblock this PR.

[stuartmorgan-g]

We've just completed the migration of the plugin code to the flutter/packages repository, as described in https://flutter.dev/go/flutter-plugins-repo-migration, and this repository is now being archived. Unfortunately that means that all in-progress PRs here must be moved to flutter/packages.

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