Expose navigateToChildGrid API in hierarchial grid

[dkamburov]

          @MayaKirova @dkamburov 

Okay, just jumping in to note that services like grid.navition are sitting public only because nobody bothered with access modifiers in the beginning and are still such because we've also had the bright idea to use some bit in docs/samples. Regardless, those are not documented or exported so using them is a workaround more than anything and exposing them further I don't think is the way to go.

Hopefully this isn't going in docs of any kind, I'd much prefer any other alternative. The Grids already have navigation API exposed (see https://www.infragistics.com/products/ignite-ui-angular/angular/components/grid/keyboard-navigation#custom-keyboard-navigation) - perhaps that can be extended to handle HGrid's more complex cases?

Originally posted by @damyanpetev in #15201 (comment)