[native] Add "Clear media cache" button to dev menu

Summary:
I use this locally to clear the media cache when I'm testing encrypted multimedia.
This is related to ENG-2745 - I just found another use case for the custom menu buttons.

This should be run as a side effect in global scope. Subsequent calls override the previous one.

Docs on this feature: https://docs.expo.dev/develop/development-builds/development-workflows/#extending-the-dev-menu

Test Plan:
Run the app and open dev menu (shake / press the m key while in metro console). The button should appear.

When clicked, the button should clear the media cache and show a toast.

Reviewers: ashoat, atul, kamil, marcin

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7966