Add accessibility.AcceleratorKey equivalent property

[harinikmsft]

There is a need to add a property - acceleratorKey to set of Accessibility APIs that apps can set for narrating the shortcut key associated with an element. This is introducing a new prop that we may want to upstream later.

Note that - we are not adding accelerator key support in the platform itself and instead letting the apps create them using basic keyboard events and hooks [#3449] . This is merely a property that apps can use to manually update the narrator.