Explicit UTF-8 encording when writing JSON/manifests

[tarilabs]

see here: https://github.com/opencontainers/image-spec/blob/2daaaaf0e7c16a6a147be91fde277f38573be672/considerations.md?plain=1#L29

in Olot code, we should explicit UTF-8 encoding when Olot writes JSON/Manifests.