load_stac_from_job: add option to require or avoid the canonical link

[soxofaan]

Connection.load_stac_from_job currently uses the canonical (signed URL) link automatically if it can find it and falls back on the standard (non-signed) job result URL.

I think it makes sense to add an option (or options) to require and/or avoid canonical links, e.g. to fail fast at client side when the canonical link can not be found while the backend would require it