Make DefaultHttpClientFactory friendly for consumers

[nvborisenko]

This is continuation of #114

We have DefaultHttpClientFactory which is used implicitly. At the same time the main consumer uses his own implementation: https://github.com/reportportal/commons-net/blob/bd375443c1e6c918720ac47587e0b4674ee6c98d/src/ReportPortal.Shared/Reporter/Http/HttpClientFactory.cs#LL44C12-L44C12

I guess 99% is similar. It would be better if the implementation can be exposed, and consumer is able to extend default implementation via inheritance.