Make ICustomPropertyProvider AOT safe

[manodasanW]

• Added BindableCustomPropertyAttribute to allow to specify that public properties from the class are bindable. Either all public properties can be bindable by passing no arguments to the constructor. Or the names of the properties and the types of the indexer types can be passed to specify which ones to make bindable.
• By adding this attribute to a class and marking it partial, an interface Microsoft.UI.Xaml.Data.IBindableCustomPropertyImplementation gets implemented on that class via the source generator which provides the implementation to support ICustomPropertyProvider. This is done as an interface instead of an attribute because this allows to support generic types while the attribute model won't work with it. In addition, we only need to worry about supporting classes and not other types.
• Added a bunch of tests to validate
• This model doesn't support built-in .NET types but those can be worked around by wrapping it around a class or so. Support for that can be added via other means if needed in the future.

partial class Language : global::Microsoft.UI.Xaml.Data.IBindableCustomPropertyImplementation
{
    global::Microsoft.UI.Xaml.Data.BindableCustomProperty global::Microsoft.UI.Xaml.Data.IBindableCustomPropertyImplementation.GetProperty(string name)
    {
        if (name == "Name")
        {
            return new global::Microsoft.UI.Xaml.Data.BindableCustomProperty(
                true,
                false,
                "Name",
                typeof(string),
                static (instance) => ((Language)instance).Name,
                null,
                null,
                null);
        }
        if (name == "Value")
        {
            return new global::Microsoft.UI.Xaml.Data.BindableCustomProperty(
                true,
                true,
                "Value",
                typeof(int),
                static (instance) => ((Language)instance).Value,
                static (instance, value) => ((Language)instance).Value = (int)value,
                null,
                null);
        }
        return default;
    }

    global::Microsoft.UI.Xaml.Data.BindableCustomProperty global::Microsoft.UI.Xaml.Data.IBindableCustomPropertyImplementation.GetProperty(global::System.Type indexParameterType)
    {
        if (indexParameterType == typeof(int))
        {
            return new global::Microsoft.UI.Xaml.Data.BindableCustomProperty(
                true,
                true,
                "Item",
                typeof(string),
                null,
                null,
                static (instance, index) => ((Language)instance)[(int)index],
                static (instance, value, index) => ((Language)instance)[(int)index] = (string)value);
        }
        return default;
    }
}

[Sergio0694]

Note: we should also make sure this works correctly with partial properties. We can handle this in a follow up.

[manodasanW]

We are using symbol data to get this data, so I believe it should. But we can confirm after.

[Sergio0694]

I was more worried about the fact you might see two different property symbols for the same member (one being the definition, one being the implementation part), and that the fact they'd be "duplicate" might cause problems.

[Scottj1s]

any plans to document this in greater detail, like with an md on the repo? Xaml Compiler could then point errors/warnings to that page.

[manodasanW]

I can get our aot.md updated to mention this in a follow up PR.

[Scottj1s]

enums (or name:value syntax) would be easier to read at call sites

[Scottj1s]

what's the purpose of passing these bools in? given they have to match customProperty, why not just use the latter? (they seem redundant)

[manodasanW]

This is validating expected vs actual. I will have a follow up PR to address this along with your enum comment for easier readability.