Setup initial App Check testapp

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

This is copied from the Storage testapp. The changes are:

• Renaming the folder Assets/Firebase/Sample/Storage -> AppCheck
• Clearing out most of the contents of UIHandler.cs and UIHandlerAutomated.cs
• Updating the Application identifier in ProjectSettings.asset
• Updating the readme

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Testing

Describe how you've tested these changes.

Building it locally

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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[AlmostMatt]

Looks ok. I wonder if readme might need some additional instructions for setting up AppCheck in the Firebase console, but that can be added after the testapp has more logic.