Add an "Underlier" interface

[thockin]

xref: go-logr/logr#11

Example:

if under, ok := log.(stdr.Underlier); ok {
    if stdLogger, ok := under.GetUnderlying().(*stdlog.Logger); ok {
        stdLogger.Printf("Hello")
    }
}

[thockin]

As per @DirectXMan12 offline