Change get_range start and end parameters to be kwarg only, add length as alternative

[kylebarron]

Many libraries use either start/end or offset/length for range requests. We should require these params to be keyword-only params.

Additionally, we should allow length as a named parameter (exclusive with end), for ease of use

This would be a breaking change. Sadly I just released 0.3, so this would require an 0.4.

[maxrjones]

FWIW I did not find that there is a significant performance cost to using dataclasses for a range requests in addition to suffix and offset requests in https://github.com/maxrjones/zarr-byterangerequest-microbenchmarks/blob/main/plot-results.ipynb, as long as you use if; then syntax rather than match; case. That was the motivation for moving to only using dataclasses in zarr-developers/zarr-python#2585. Norman suggested this possible design for supporting length as well. It would be simple to run similar tests specifically for obstore if helpful.

But, requiring kwargs would similarly avoid confusion.