Allowed PhysicalFile attributes to be set using known attribute names

[reduckted]

Relates to #312.

I've added an overload to PhysicalFile .TrySetAttributeAsync where you can pass in an enum value instead of a string for the property name. This allows unusual attribute names (such as {ItemType}, which maps to the build action) to be easily specified.

I've also changed the value parameter to be an object, which allows bool or enum values to be passed in rather than having the caller convert the value to a string.

Examples:

await file.TrySetAttributeAsync(PhysicalFileAttribute.BuildAction, "None");

await file.TrySetAttributeAsync(PhysicalFileAttribute.CopyToOutputDirectory, CopyToOutputDirectory.Always);

await file.TrySetAttributeAsync(PhysicalFileAttribute.Visible, false);

await file.TrySetAttributeAsync(PhysicalFileAttribute.CustomToolNamespace, "Test");

I got the list of attributes by getting the property descriptors from a C# file (using the same code that's used in TrySetAttributeAsync), and ignoring any that were read-only.

[madskristensen]

That is absolutely awesome. Thank you so much!!