multiple update domains and/or fallback domains

[chirayudesai]

The update is currently downloaded from a single source, and if for some reason that source goes down updates will not be available at all.

It would be good to have multiple sources supported, and this could also work like a mirror ( (like how F-Droid does it). Check the default server first (with multiple timeouts), and if that fails fallback to a different source. There can be only one backup source, but the code difference for supporting multiple backup sources wouldn't be too huge so it would make sense to just go for that I think.

[thestinger]

It should randomly shuffle the array to spread out the load evenly.

[thestinger]

GrapheneOS now uses round-robin DNS for releases.grapheneos.org and this could use active health checks. It might make sense to support another domain in case DNS goes down but it's not particularly important.

[thestinger]

releases.grapheneos.org now uses GeoDNS and round-robin within each GeoDNS region. It would still make sense to have a fallback domain but most of this is being handled via https://github.com/GrapheneOS/ns1.grapheneos.org.

The seamlessupdate.app domain is ready to be used as a fallback domain. Client needs to have that implemented.