[Feature request] Add function get_element_html(bid)

Plase add a new function in [functions.py](https://github.com/ServiceNow/BrowserGym/blob/main/browsergym/core/src/browsergym/core/action/functions.py) to return the HTML of a element identified by its `bid`.

This is for https://github.com/All-Hands-AI/OpenHands/issues/5727#issuecomment-2603470848