Implement receiver_buffer_size kwarg

[anecdata]

Implement receiver_buffer_size kwarg to match busio:
https://docs.circuitpython.org/en/latest/shared-bindings/busio/index.html#busio.UART

Ideally using background read so the library doesn't block when awaiting (larger) chunks of data.

Motivation: PIO FIFOs are very limited in size. Lower baudrates and / or hardware flow control can help in some use cases.