config: allow exposing real secret value through marshal

[GiedriusS]

There are external projects out there that marshal/unmarshal the Prometheus config that has secrets. To get the real value currently such horrible workarounds are in place: https://github.com/tkestack/kvass/blob/master/pkg/sidecar/injector.go#L175-L214

Let's add a way to get the original values through the Secret type to avoid such things.

[SuperQ]

Good idea, minor nit about the test setup.

[alita1991]

Is there any plan to merge the code in the next period of time?

[roidelapluie]

thanks, I think we can do the test improvement later on.