Add user agent

[StephenWeatherford]

microsoft/vscode-azuretools#200

[PrashanthCorp]

Need to verify that it's fully implemented.

[PrashanthCorp]

Currently among the clients we can create, we only create a StorageManagementClient. I verified that we use createAzureClient<T> whenever we create a client of type T. This directly adds the extension user agent. This is similar to how other extensions do it.

Closing this.

Question: This feels like a probable issue every time there's new code using an azure client. If the author forgets to wrap it in a createAzureClient call, we would not have the user agent info.