Add support for relative href links

[GorgiAstro]

My use case: I use the STAC FastAPI server to serve the STAC catalog, collection & items. The assets are stored and served from the same server, so I use a relative link in the href field of the asset, while the collection contains the absolute server root URI. Relative href links are supported in the STAC spec, and I managed to download sucessfully the asset from the STAC browser. However it does not look like relative asset links are supported by the QGIS STAC plugin. Looking at the source code and adding a few print, the assets' relative href is never resolved using the root href URI.