[vector-api] ol.format.Format#readFeaturesFromURL

This PR moves XMLHttpRequest handling into the `ol.format.Format` class, allowing all formats to support reading from URLs. This can be useful for formats that only contain a single feature or geometry (e.g. WKT, WKB, Polyline, IGC) for which `ol.source.VectorFile` does not make much sense. I'm not sure if this PR is a good thing or not.