Optimize the configuration files for using Feathr in an online setting

When end users use Feathr online inference (such as in AKS or ACI), they still need to setup Feathr client, but in a lighter way (i.e. they only need redis credentials), without even a configuration file. We should enable that scenario. This should have a few parts:
- if something is missing (say purview or offline config), the initialization should still be completed, and only initialize the parts that are configured.