Split Notification utility helpers into a mutiple purpose-focused files

[danielayala94]

Describe the bug

Currently we have an AppNotificationUtility which contains different helper functions addressing unrelated areas.

We want a common folder which hosts mutiple purpose-focused files containing different sets of helpers.

Steps to reproduce the bug

Code bug

Expected behavior

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Screenshots

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NuGet package version

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Packaging type

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Windows version

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IDE

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Additional context

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[danielayala94]

Also, ConvertUtf8StringToWideString can be moved to the WinAppSDK common Utf8 utility file and maybe add a wil::unique_cotaskmem_string overload.