feat(ui_shared): pass platform-specific icons and colors explicitly

[lesnitsky]

Description

Allows to pass cupertinoColor, materialColor, cupertinoIcon and materialIcon to the UniversalButton and LoadingButton instead of checking isCupertino in the parent each time

Related Issues

none

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.

This is technically a breaking change, but firebase_ui_shared wasn't released yet.

[Lyokone]

Shouldn't it be a feat: instead of chore:?