[msbuild] Show a better error with invalid app bundles due to Resources subdirectory. Fixes #20135.

[rolfbjarne]

iOS (and presumably tvOS) app bundles can't contain a subdirectory named "Resources".

Apple says:

Note: An iOS app bundle cannot include a custom folder named “Resources.”

Ref: https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFBundles/BundleTypes/BundleTypes.html#//apple_ref/doc/uid/10000123i-CH101-SW1

Unfortunately Apple's toolchain won't show a helpful error message, the
eventual failure is codesign saying something like this:

bin/Release/net8.0-ios/ios-arm64/SomeApp.app: code object is not signed at all
In subcomponent: bin/Release/net8.0-ios/ios-arm64/SomeApp.app/System.Security.Cryptography.aotdata.arm64

Which is confusing, to say the least.

After debugging this myself a few times, and seeing customers running into the
same issue periodically, it's time to make the error more actionable.

I've added code to our Codesign task to detect the "Resources" subdirectory,
and show a better error message. There's also a way to disable the validation
(by setting CodesignDisallowResourcesSubdirectoryInAppBundle=false), just in
case we end up being overeager with the validation.

Fixes #20135.

[vs-mobiletools-engineering-service2]

🔥 Unable to find the contents for the comment: D:\a\1\a\change-detection\results\gh-comment.md does not exist :fire

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✅ dotnettests (iOS): All 1 tests passed. Html Report (VSDrops) Download
✅ dotnettests (MacCatalyst): All 1 tests passed. Html Report (VSDrops) Download
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