Update to support S3-compatible storage

[djmitche]

GothenburgBitFactory/taskchampion#601 adds functionality to TaskChampion to allow sync to services that act like S3, such as Minio or Ceph. With #4019, this is now available to Taskwarrior, but not yet supported in TW's configuration or documented.

[djmitche]

@travisby are you interested in working on this? We'll need to wrap this up to release 3.5.0.