Enable /await opt-out so users can manually enable /await:strict. (#1015)

* Switch to using /await:strict which is binary compatible with c++20 coroutines.

* Make legacy coroutines opt-in.

nuget/CppWinrtRules.Project.xml

@@ -81,4 +81,9 @@
                 Description="Enables or disables the default for copying binaries to the output folder to be false"
                 Category="General" />
 
+  <BoolProperty Name="CppWinRTEnableLegacyCoroutines"
+                DisplayName="Enable legacy coroutines (C++17)"
+                Description="Enables the /await compiler option (disable this if you want to pass this yourself)"
+                Category="General" />
+
 </Rule>

nuget/Microsoft.Windows.CppWinRT.targets

@@ -871,7 +871,7 @@ $(XamlMetaDataProviderPch)
     <ItemDefinitionGroup>
         <ClCompile>
             <AdditionalOptions>%(AdditionalOptions) /bigobj</AdditionalOptions>
-            <AdditionalOptions Condition="'%(ClCompile.LanguageStandard)' == 'stdcpp17'">%(AdditionalOptions) /await</AdditionalOptions>
+            <AdditionalOptions Condition="'%(ClCompile.LanguageStandard)' == 'stdcpp17' And '$(CppWinRTEnableLegacyCoroutines)' != 'false'">%(AdditionalOptions) /await</AdditionalOptions>
             <AdditionalIncludeDirectories>%(AdditionalIncludeDirectories);$(GeneratedFilesDir)</AdditionalIncludeDirectories>
         </ClCompile>
         <Midl Condition="'$(CppWinRTModernIDL)' != 'false'">