shell: backends: uart: Rework and add support for async API

Rework UART backend to clearly use single instance approach as
before it was a bit messy with some parts of implementation indicating
multi-instance approach and some single instance. Backend has been
around for years and multi-instance requirement never came.

Added support for UART asynchronous API which is more efficient
in terms of power consumption and performance. Asynchronous API
support is using uart_async_rx helper module for handling data
received asynchronously.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>