Provide a way to understand the effective config and the supplying source

[Emily-Jiang]

As an operations person, I want to have an easy way to read out what configured values can be tweaked by the operation team.
Having an application configurable is only the first step. Only having an ops team which can easily query which ConfigSources got picked up in which order.

[Emily-Jiang]

We would provide an api to return the effective config source so that implemenation can utilise this info and build on top of it for operation team to query. It is also useful for developer to debug configuration as well during testing.