Provide base route

A toolkit to address https://github.com/7digital/SevenDigital.Api.Wrapper/issues/36 - "Enable using different API urls for each endpoint"
May want to include all or some of this.

There may be other ways to do this, e.g.
private string GetApiBaseUrl(RequestData requestData)
{
  return _baseUriProvider.BaseUri(requestData);
}

where there is a default version of baseUriProvider that uses _apiUri as below. 
This would give more visibility to the RequestData class, which only used internally at present.