bluetooth: buf: Add a callback for freed buffer in rx pool

The Bluetooth data buffer API currently lacks a mechanism to notify when
a buffer is freed in the RX pool. This limitation forces HCI drivers to
adopt inefficient workarounds to manage buffer allocation.

HCI drivers face two suboptimal options:

- Blocking calls: Use bt_buf_get_rx with K_FOREVER, which blocks the
  execution context until a buffer becomes available.
- Polling: Repeatedly call bt_buf_get_rx with K_NO_WAIT, which increases
  CPU load and reduces efficiency.

This commit introduces a callback mechanism that is triggered each time
a buffer is freed in the RX pool. With this feature, HCI drivers can:

- Call bt_buf_get_rx with K_NO_WAIT.
- Wait for the callback notification if a NULL buffer is returned,
  avoiding unnecessary polling.

The new callback improves efficiency by enabling event-driven behavior
for buffer management, reducing CPU overhead while maintaining
responsiveness.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>