gclue-modem-manager: support assistance data servers

ModemManager may expose assistance data servers if the modem GNSS engine supports it. These servers provide GNSS assistance data to get a faster GNSS fix, tailored towards the GNSS engine. Check if assistance data servers are supported by the GNSS engine, fetch the list of servers, download the assistance data, and inject it into the GNSS engine before enabling the GNSS engine. Users may set an assistance server in the GeoClue config file for privacy reasons as the default assistance servers may collect metadata. This way, users can use alternative servers if they want to. By default the assistance servers provided by the modem firmware are used.

Tested on the Pine64 PinePhone with postmarketOS.

Note: I split off the port to libsoup3 to make it easier to backport this patch to GeoClue 2.6.X releases. This port patch can be squashed together with the original patch.