Support adding assets with s3:// protocol

[rockliffelewis]

Some STAC catalogs (eg. DEA) make use of s3:\\ style links to s3 buckets, and it would be easy to translate those to /vsis3/ links to engage the S3 capabilities of GDAL to add the assets to the QGIS project.

You dont need to worry about authentication, as that can all be handled by GDAL.

[drnextgis]

@rockliffelewis I think that this pull request resolves the issue.