shell: add generic RX buffer flush function

Added a generic function `z_shell_backend_rx_buffer_flush` to clear
the RX buffer when resuming the shell. The function repeatedly calls
the backend's `read` API until the buffer is empty or a maximum of
1000 iterations is reached. This prevents unintended command
execution after `shell_start`.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@verkada.com>