App-configurable compatibility options implementation: CompatibilityOptions

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Implements #4966.

Adds CompatibilityOptions

CompatibilityOptions configures which changes in Windows App SDK servicing releases are enabled. By default, all changes are enabled, but CompatibilityOptions can be used to lock the runtime behavior to a specified patch version or to disable specific changes:

• Choose the patch version: You can specify which servicing patch version's behavior you want to use. For example, your app can specify that it wants the 1.7.2 version behavior, which will have Windows App SDK run in that mode even if 1.7.3 or later is installed. This capability allows you to control when your app gets new fixes or behavior changes, even when not using self-contained mode.
• Temporarily disable specific changes: If your app encounters a problem with a specific change in a servicing update, you can disable just that change while still benefiting from the other changes or features in that update. (All changes are enabled by default for the patch version in use.) Disabling a change is a temporary measure which gives time for a fix to be released in a future Windows App SDK update or for you to implement an update in your app.

Implementation notes

• Per the spec, project properties can be set rather than directly using CompatibilityOptions. This works by generating an auto-initializer with the specified options.
• The existing auto-initializers have been adjusted to be helpers called by a single, common auto-initializer. This ensures a clear and consistent order of initialization.
• A few core CompatibilityTests have been added for the new API and are included in the test suite.
• Test apps for both C# and C++ have been added to test the project properties. These tests are just compile-time tests to ensure the properties generate the correct code. These test apps don't install or bootstrap the runtime to be able to actually run the code.

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