Need to perform format validations on host.json / function.json

Currently we read this json file and make assumptions that content is valid. We need to do proper validation for all values.

The issue names host.json and function.json but no source files, tests, or schema. Start by locating where the host reads these JSON files, then determine the expected validation rules and how invalid values should be reported. Done means both file formats are validated instead of being accepted on assumption.

Written by the indexing model from the issue text.