h3i: add interactive mode to async client

Add H3iHandle and connect_interactive() to allow submitting actions
one at a time via a channel, rather than providing all actions upfront.

Changes:
- Add H3iHandle struct with do_action() and finish() methods
- Add connect_interactive() function that returns an H3iHandle
- Refactor H3iDriver to use VecDeque action queue instead of Vec+index
- Add action_rx channel for receiving actions in interactive mode
- Move action processing from iterator-based to queue-based approach
- Process incoming actions in work_loop via select! macro

This enables use cases where actions need to be submitted dynamically
based on server responses, rather than predetermined at connection time.

Author: evanrittenhouse