Suggestion: Enable optional customized http.Client for transmitter

[wmdave]

I'm building an application which leverages the app insights go client and, in order to run the resulting application in a minimal container (i.e. one without an OS CA certificate store), I need the ability to programmatically include trusted CA certificates in the transmitter's http.Client.

To enable this, I was thinking of adding the option to include a *http.Client option in TelemetryConfig which can then be leveraged during Transmit to call the app insights endpoint. If the value of the TelemetryConfig.Client option is nil, the call to newTransmitter would substitute the http.DefaultClient automatically.

[jjjordanmsft]

This was merged. Thanks again.