Add Buffer.as_buffer_like method.

[kylebarron]

The core Buffer class represents a block of memory in either CPU or GPU memory.

There's no current method on Buffer to expose the memory via the buffer protocol without always forcing a copy. The closest thing is the as_array_like method, which returns an ArrayLike, but there's no way to get a BytesLike from it.

So what I want is

    @abstractmethod
    def as_bytes_like(self) -> collections.abc.Buffer:

(#2814 tracks the unification of BytesLike and collections.abc.Buffer types)

In practice, this means that the obstore integration (#1661) won't need to force a memory copy when uploading data.

ref #1661 (comment), cc @TomAugspurger