Refactor HTTP request handling in `SproxydFileSystem`
Refactor HTTP request handling in `SproxydFileSystem`

In several methods of the `SproxydFileSystem` class, a similar pattern
of performing an HTTP request and handling the response was used. This
commit abstracts out this common pattern, which simplifies code and
should ease unit-testing.

scality/ScalitySproxydSwift,scality/ScalitySproxydSwift