* Adding the user credentials exec abillity
new file: src/KubernetesClient/KubeConfigModels/ExecCredentialResponse.cs
new file: src/KubernetesClient/KubeConfigModels/ExternalExecution.cs
modified: src/KubernetesClient/KubeConfigModels/UserCredentials.cs
modified: src/KubernetesClient/KubernetesClientConfiguration.ConfigFile.cs
* Fixed a few issues with the process spawning and some null references issues
* Removed unused import that caused the build to fail (Mail)
* Added preprocessor directive that will disable out-of-tree client authentication in case it is not a asp.net core app
* Added tests to the new external execution (out-of-tree client authentication) extension
* Trying to fix failing tests that fail apparently due to the preprocessor symbol
* Trying to fix failing macos tests
* Added the -n (do not output trailing newline) and the -E options to the echo command in OSX
* initializing arguments variable
* Changes according to tg123 comments
Changed OSX testing command to printf to try and solve the JSON
parsing errors
* Added missing references
* Environment.UserInteractive and Process applies to .NET Standard >= 2.0 according to Microsoft documentation
