[BottomAppBar] BottomAppBar doesn't allow Floating Action Buttons to nestle further down into the bar beyond the center of the FAB.

Nobody has claimed this yet.

**Is your feature request related to a problem? Please describe.**
BottomAppBar uses `fabCradleVerticalOffset` to allow developers to set a vertical offset which controls how "deep" the FAB may nestle into the cradle of the BottomAppBar. However, there is a hard limit at 0 which equates to the radius of the FAB. So, there is no way for a FAB to nestle further down into the app bar even if the cradle margin (how large the cradle is) is deeper than the radius of the FAB.

**Describe the solution you'd like**
I'm proposing this hard limit be removed so that FABs can nestle further into the BottomAppBar. A negative `fabCradleVerticalOffset` should be allowed or the baseline value of `fabCradleVerticalOffset` should be redefined so FABs can be set further down into the BottomAppBar (closer to the bottom edge).

**Describe alternatives you've considered**
The only alternative I have is to copy BottomAppBar and create my own BottomAppBar that supports this, but I don't want to do this because then I'd lose out on future updates to the BottomAppBar API.

**Additional context**
2 images here for context:
1st is an image of the max limit of how far down a FAB can go
2nd is an image of what would be possible with this proposal
https://imgur.com/a/3aRQ5xK

Start by locating the BottomAppBar implementation and the fabCradleVerticalOffset handling. Compare the current lower-bound behavior with the requested negative-offset behavior, then identify the relevant component tests or examples. Done means a FAB can nestle farther into the bar while existing BottomAppBar behavior remains intact.

Written by the indexing model from the issue text.