fix: export platform_interface

[jamieastley]

What this does

• exports flutter_branch_sdk_platform_interface so as to consume from outside this package

Why this is required

• As per the plugin_platform_interface docs, exporting this class allows users of this package to fake/mock the Branch SDK instance in tests and validate their own business logic.

[jamieastley]

Apologies, just realised this is to merge to master, but not sure if dev would be preferred given there's no CONTRIBUTING.md defined.

Let me know which is preferred and I'll update accordingly.

[RodrigoSMarques]

Hi @jamieastley
You can keep it in the master branch.

I will merge soon and release a new version