Add support for @(InternalsVisibleTo) items that turn into assembly attributes

Given that `InternalsVisibleTo` (IVT) is such a common assembly-level attribute, this adds support
for specifying it directly via simple items, such as:

```
  <ItemGroup>
    <InternalsVisibleTo Include="MyLibrary.Tests" />
  </ItemGroup>
```

Optionally, a `Key` metadata can be specified to provide a strong-named IVT:

```
  <ItemGroup>
    <InternalsVisibleTo Include="MyLibrary.Tests" Key="PUBLIC_KEY" />
  </ItemGroup>
```

The targets will also use automatically a `$(PublicKey)` if available and no `%(Key)` metadata
override is found. Otherwise, it will default to an IVT without a key.

This also avoids having to learn the `_Parameter1` syntax in `AssemblyAttribute` elements, and is
more similar to the way other higher-level properties like `AssemblyTitle` or `Product` are also
turned into assembly attributes.

To turn off this feature, set `$(GenerateInternalsVisibleToAttributes)` to `false`.

Partially fixes #3166

Author: kzu

src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.GenerateAssemblyInfo.targets

@@ -34,6 +34,7 @@ Copyright (c) .NET Foundation. All rights reserved.
     <GenerateAssemblyVersionAttribute Condition="'$(GenerateAssemblyVersionAttribute)' == ''">true</GenerateAssemblyVersionAttribute>
     <GenerateNeutralResourcesLanguageAttribute Condition="'$(GenerateNeutralResourcesLanguageAttribute)' == ''">true</GenerateNeutralResourcesLanguageAttribute>
     <IncludeSourceRevisionInInformationalVersion Condition="'$(IncludeSourceRevisionInInformationalVersion)' == ''">true</IncludeSourceRevisionInInformationalVersion>
+    <GenerateInternalsVisibleToAttributes Condition="'$(GenerateInternalsVisibleToAttributes)' == ''">true</GenerateInternalsVisibleToAttributes>
   </PropertyGroup>
 
   <!-- 
@@ -94,6 +95,11 @@ Copyright (c) .NET Foundation. All rights reserved.
       <AssemblyAttribute Include="System.Resources.NeutralResourcesLanguageAttribute" Condition="'$(NeutralLanguage)' != '' and '$(GenerateNeutralResourcesLanguageAttribute)' == 'true'">
         <_Parameter1>$(NeutralLanguage)</_Parameter1>
       </AssemblyAttribute>
+      <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo" Condition="%(InternalsVisibleTo.Identity) != '' and '$(GenerateInternalsVisibleToAttributes)' == 'true'">
+        <_Parameter1 Condition="'%(InternalsVisibleTo.Key)' != ''">%(InternalsVisibleTo.Identity), PublicKey=%(InternalsVisibleTo.Key)</_Parameter1>
+        <_Parameter1 Condition="'%(InternalsVisibleTo.Key)' == '' and '$(PublicKey)' != ''">%(InternalsVisibleTo.Identity), PublicKey=$(PublicKey)</_Parameter1>
+        <_Parameter1 Condition="'%(InternalsVisibleTo.Key)' == '' and '$(PublicKey)' == ''">%(InternalsVisibleTo.Identity)</_Parameter1>
+      </AssemblyAttribute>
     </ItemGroup>
   </Target>