Add `flush_rx_buffer` to `BusABC`

[grant-allan-ctct]

Is your feature request related to a problem? Please describe.

This request relates to problems that can occur for certain hardware/OS combinations when users try to filter the CAN messages coming through the bus. If the interface in use supports driver-level filtering, and if the driver does not take steps to apply the filter to (or simply purge) already-received messages whenever a new filter is configured, then the caller of set_filters may end up with unwanted messages at the beginning of their collection, once they start collecting received messages. (Examples: #1708 and #1413.)

Describe the solution you'd like

Even aside from the problem mentioned, I think it would be nice for a user of an interface to have a flush_rx_buffer facility that's a partner to the flush_tx_buffer already present in the BusABC interface definition.

Depending on circumstances, this may allow a user to avoid importing a lower-level python library specifically to accomplish that flushing a different way.

Describe alternatives you've considered

Additional context

I think that it might make sense to have two different phases to this work (e.g. two different PRs if not two different Issues).

1. Add flush_rx_buffer to the BusABC class and provide implementations for (some of) the existing interfaces. This will help users put workarounds in place for situations like #1708 without resorting to lower-level techniques like used in #1413. It's also good to have this facility in the interface for general purpose usage. Doing this up-front also allows developers of emerging interfaces to join the party early.
2. Devise the best way to integrate flush_rx_buffer in with the Bus.set_filters to prevent future problems of a similar nature to #1708 and #1413.

[ilanbiala]

+1 to this, for example when using pytest with python-can fixtures, the CAN bus is already initialized and flushing old messages would be ideal rather than having to do something like an empty for-loop to just pass on eating through all the messages.