[C++] Vendor a span implementation

[asfimport]

std::span is a basic vocabulary type which won't be available to us until c++20. We should vendor an implementation similar to our util::optional

Reporter: Ben Kietzman / @bkietz

_{Note: This issue was originally created as ARROW-14083. Please see the migration documentation for further details.}

[bkietz]

A span polyfill was added in #36334