Add a copilot package icon to the .NET nuget package (#688)

Author: stephentoub

assets/copilot.png

@@ -11,6 +11,7 @@
         <PackageProjectUrl>https://github.com/github/copilot-sdk</PackageProjectUrl>
         <PackageReadmeFile>README.md</PackageReadmeFile>
         <RepositoryUrl>https://github.com/github/copilot-sdk</RepositoryUrl>
+        <PackageIcon>copilot.png</PackageIcon>
         <PackageTags>github;copilot;sdk;jsonrpc;agent</PackageTags>
         <IsAotCompatible>true</IsAotCompatible>
         <IncludeSymbols>true</IncludeSymbols>
@@ -25,6 +26,7 @@
 
     <ItemGroup>
         <None Include="../README.md" Pack="true" PackagePath="/" />
+        <None Include="../../assets/copilot.png" Pack="true" PackagePath="/" />
     </ItemGroup>
 
     <ItemGroup>