[msbuild] Add support for 'EnableCodeSigning' for mobile projects. (#14379)

Mac Catalyst and macOS projects support an 'EnableCodeSigning' property to determine
whether an app is signed or not. In order to bring parity on mobile platforms, add
support for this property for iOS, tvOS and watchOS projects as well - if not set,
we'll still have the old behavior of signing device builds and not signing simulator builds.

Co-authored-by: Manuel de la Pena <mandel@microsoft.com>

msbuild/Xamarin.Shared/Xamarin.Shared.props

@@ -145,8 +145,12 @@ Copyright (C) 2020 Microsoft. All rights reserved.
 	<!-- RequireCodeSigning -->
 	<!-- iOS/watchOS/tvOS is simple: device builds require code signing, simulator builds do not. This is a big lie, for some simulator builds need to be signed, but the _DetectCodeSigning task handles those cases. -->
 	<PropertyGroup Condition="'$(_PlatformName)' != 'macOS' And '$(_PlatformName)' != 'MacCatalyst'">
-		<_RequireCodeSigning Condition="'$(_RequireCodeSigning)' == ''">false</_RequireCodeSigning> <!-- Xamarin.iOS builds are not signed by default -->
-		<_RequireCodeSigning Condition="'$(ComputedPlatform)' == 'iPhone'">true</_RequireCodeSigning> <!-- except that device builds must be signed -->
+		<!-- Make it possible to override the default logic by setting EnableCodeSigning -->
+		<_RequireCodeSigning Condition="'$(_RequireCodeSigning)' == ''">$(EnableCodeSigning)</_RequireCodeSigning>
+		<!-- Device builds must be signed -->
+		<_RequireCodeSigning Condition="'$(_RequireCodeSigning)' == '' And '$(ComputedPlatform)' == 'iPhone'">true</_RequireCodeSigning>
+		<!-- Otherwise code signing is disabled by default (simulator builds)-->
+		<_RequireCodeSigning Condition="'$(_RequireCodeSigning)' == ''">false</_RequireCodeSigning>
 	</PropertyGroup>
 	<!-- macOS is a bit more complicated:
 		* 'EnableCodeSigning' specifies whether the app is signed or not, and this defaults to false if it's not set.