Ability to customize client engine

In 3.0.19 we had an @Override of the ResteasyClientBuilder#build() method. This already convoluted workaround was necessary to configure Apache HTTP client, for example, to add a request interceptor.

Such configuration is now effectively impossible after this commit:

https://github.com/resteasy/Resteasy/commit/bbdf28597bb9367fcc75268d2c8abcc99b74a80c#diff-4913de17c695a002c91eeafdd0c7d7b3

The new build() method uses private fields, so it can't be duplicated. The HttpClientBuilder43.initDefaultEngine43() call is protected static. The ResteasyClient constructor is package visible. Code that was already bad is now much worse.