FR (Liveness) Callback to listen for interactions with Begin Check button

### On which framework/platform would you like to see this feature implemented?

React, iOS, Android

### Which UI component is this feature-request for?

Other

### Please describe your feature-request in detail.

We would like to have a callback to listen for the user interactions with "Begin Check" button. This would be something along the lines of `onUserCancel` callback we have today.

This gives us a way to understand user behavior around component usage for example. number of user started the check but did not complete etc.

### Please describe a solution you'd like.

A callback like `onBeginCheck` would be a potential way forward.

### We love contributors! Is this something you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request.
- [ ] ⚠️ This feature might incur a breaking change.

Start by locating the Liveness component implementations and the "Begin Check" button in React, iOS, and Android. Read the existing onUserCancel callback behavior first. Done means a documented callback consistently reports user interaction with "Begin Check" across the requested platforms.

Written by the indexing model from the issue text.