Improve mocking capabilities

[Donnerbart]

Is your enhancement related to a problem? Please describe

It's hard to mock pods/exec calls for other stream IDs than stdout:

server.expect()
    .withPath("/api/v1/namespaces/test/pods/pod1/exec?command=ls&container=debug&stdout=true")
    .andUpgradeToWebSocket()
    .open(new OutputStreamMessage(expectedOutput))
    .done()
    .always();

Describe the solution you'd like

The client should provide ErrorStreamMessage and StatusStreamMessage to support mocking like:

server.expect()
  .withPath("/api/v1/namespaces/test/pods/pod1/exec?command=ls&container=default&stderr=true")
  .andUpgradeToWebSocket()
  .open(new ErrorStreamMessage(expectedError))
  .done()
  .always();

server.expect()
  .withPath("/api/v1/namespaces/test/pods/pod1/exec?command=ls&container=default&stdout=true")
  .andUpgradeToWebSocket()
  .open(new StatusStreamMessage(1))
  .done()
  .always();

Describe alternatives you've considered

I've written the classes for our project, but this needs some code duplication, because we cannot re-use the private method getBodyBytes() from OutputStreamMessage.

Additional context

No response