Comment 0 for bug 1881173

The 'custom_registries' config option is a json array posing as a string. This is prone to error, so we should validate that the data contains the fields and data types that we expect.

Initially discussed in https://github.com/charmed-kubernetes/charm-containerd/pull/39, one example of things we need to validate is that 'ca|cert|key_file' values are b64 encoded. Another would be that each registry item has a 'url' key/value.