fix: include net http package on compatibility test

Author: Justintime50

EasyPost.Compatibility.NetStandard/EasyPost.Compatibility.NetStandard.csproj

@@ -22,6 +22,7 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
+    <PackageReference Include="System.Net.Http" Version="4.3.4" />
   </ItemGroup>
 
 </Project>