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

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

```
    <AssemblyMetadata Include="Foo" Value="Bar" />
    <AssemblyMetadata Include="Bar" Value="Baz" />
```

This also avoids having to learn the `_Parameter1` and `_Parameter2` 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.

Partially fixes #3166

Author: kzu

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

@@ -94,6 +94,10 @@ Copyright (c) .NET Foundation. All rights reserved.
       <AssemblyAttribute Include="System.Resources.NeutralResourcesLanguageAttribute" Condition="'$(NeutralLanguage)' != '' and '$(GenerateNeutralResourcesLanguageAttribute)' == 'true'">
         <_Parameter1>$(NeutralLanguage)</_Parameter1>
       </AssemblyAttribute>
+      <AssemblyAttribute Include="System.Reflection.AssemblyMetadata" Condition="%(AssemblyMetadata.Identity) != ''">
+        <_Parameter1>%(AssemblyMetadata.Identity)</_Parameter1>
+        <_Parameter2>%(AssemblyMetadata.Value)</_Parameter2>
+      </AssemblyAttribute>      
     </ItemGroup>
   </Target>