Read the queryables href from the catalog

[rockliffelewis]

I have a queryables href set as part of my catalog definition which is different from the usual [catalog_href]/queryables

Like it is defined in the example here: https://github.com/stac-api-extensions/filter#interaction-with-endpoints

The QGIS STAC Plugin doesn't appear to be reading this, and is instead just assuming that [catalog_href]/queryables is the correct url to fetch.