fix(secure_storage): Add plugin platform entry.

[Equartey]

Issue #, if available:
#5165
Description of changes:
Followed Flutter doc guidance to specify platform specific entry points for the Secure Storage plugin. This resolves the build errors from Flutter Master channel:

Package amplify_secure_storage:linux references amplify_secure_storage_dart:linux as the default plugin, but the package does not exist.
Ask the maintainers of amplify_secure_storage to either avoid referencing a default implementation via `platforms: linux: default_package: amplify_secure_storage_dart` or create a plugin named amplify_secure_storage_dart.

Package amplify_secure_storage:windows references amplify_secure_storage_dart:windows as the default plugin, but the package does not exist.
Ask the maintainers of amplify_secure_storage to either avoid referencing a default implementation via `platforms: windows: default_package: amplify_secure_storage_dart` or create a plugin named amplify_secure_storage_dart.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[Equartey]

Can this can be a no-op? There isn't a instance from the current impl to map here.

[Jordan-Nelson]

default_package would typically refer to another package. This references itself. Is that something flutter docs suggest doing?

What I am seeing in the docs is to use dartPluginClass

Suggested change

	default_package: amplify_secure_storage_dart
	dartPluginClass: AmplifySecureStorageDart

You will also have to add a registerWith() method to AmplifySecureStorageDart I believe. This is based on https://docs.flutter.dev/packages-and-plugins/developing-packages#dart-only-platform-implementations