Enhancement: Add option to disable AWS S3 dual-stack endpoints

Restic used dual-stack endpoints for AWS regions that have them, with no way to
opt out. VPC interface and gateway endpoints do not support dual-stack, so a
repository reachable over a VPC endpoint could not be used.

Passing `-o s3.disable-dualstack=true` now keeps restic on the single-stack
endpoint.

https://github.com/restic/restic/issues/5189
https://github.com/restic/restic/pull/22060
