datamodel: validation: check that directory/file is readable/writable for kresd

During validation, we check for the existence of directories/files in the configuration, but they may be unreadable/unwritable for kresd and the resolver can fail to start even configuration is valid. Then it may be unclear where the error actually is.

The solution may be to create custom types (e.g. ReadableFile, WritableDir) that additionally check the rights for kresd.