Enable some of the unnecessary code rules (#799)

* CA1823 Avoid unused private fields
* CA1847 Use string.Contains(char) instead of string.Contains(string) with single characters
* IDE0005 Remove unnecessary import
* IDE0051 Private member is unused
* IDE0052 Private member is unread

Directory.Build.props

@@ -2,5 +2,7 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
     <CodeAnalysisRuleSet>$(MSBuildThisFileDirectory)\kubernetes-client.ruleset</CodeAnalysisRuleSet>
+    <EnableNETAnalyzers>true</EnableNETAnalyzers>
+    <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
   </PropertyGroup>
 </Project>