[path_provider] Path provider platform interface

[TimWhiting]

Description

This PR is the first step in converting path_provider to use a federated plugin implementation.
It follows the first step in this article for creating federated plugins.
Though it's not determined yet what web should do for this package flutter/flutter #45296, I'm interested in creating a Linux implementation in Dart rather than C++ see google/flutter-desktop-embedding #674, and a federated interface would help with that.

Related Issues

Related: flutter/flutter#45296
Related: google/flutter-desktop-embedding#674

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• 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 signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[matthew-carroll]

I'm gonna switch to @amirh as the reviewer here because I'm not yet familiar with the plans for federated plugins.