set user agent header

The topic of usage metrics came up, I think we should override https://github.com/containers/container-libs/blob/a3b0f19c3ea12cb02f0b11788489c52d4fb02e96/image/internal/useragent/useragent.go#L6 (vs the skopeo proxy).

That'd allow registries to distinguish "image pulls for build" vs "image pulls for bootc client runs".