Support dedicated providers

[jiangpengcheng]

in our downstream, we provide OpenWhisk as a FaaS service, and now we want to provide kafka feeds features to users, we can use current code to run some kafka feed providers for users but it cannot satisfy below requirements:

1. users' kafka brokers may behind a firewall which block requests from kafka feed providers maintained by us
2. if user's kafka produce massive data, our common kafka feed providers may not be able to handle these data quickly

so this pr provides a way to make user can use their own feed providers

[jiangpengcheng]

yes, I have tested this and it worked as expected

[style95]

@dgrove-oss Thanks for the confirmation!