[file_selector] Add getDirectoriesPaths method to the file_selector_platform_interface.

[eugerossetto]

This PR adds the implementation of the method getDirectoriesPaths to the file_selector_platform_interface.

Next, we will proceed this way:

1. Add implementations for Windows, macOS and Linux in parallel. (The Windows implementation will be done when the Dart migration is ready)
2. Add the new published version to the file_selector package with its example when at least one platform is published.
3. Repeat 2 when the other new implementations are published.

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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[adpinola]

Looks good, I'm wondering if we should add the method to the MethodChannelPlatformInterface implementation that is available in lib/src/method_channel/method_channel_file_selector.dart

I'm not sure why do we need that at all.

[adpinola]

just a minor nit
🚀

[adpinola]

Suggested change

	* Adds `getDirectoriesPaths` method to interface.
	* Adds `getDirectoriesPaths` method to platform interface.