Formats: privacy considerations

We need to investigate what the privacy implications are of each model format we will recommend. The model formats themselves can fetch resources, so we need to put a privacy and security framework around what schemes they can fetch (https only, for instance). We also need to say what all the fetch policies are. Need to investigate if the formats provide any guidance here, or if they leave it up to the implementation. If they do, we need to specify it (i.e., don't send cookies, don't leak the referrer, etc.).

Start by comparing the recommended model formats' resource-fetch behavior and any privacy or security guidance they provide. Define the supported fetch schemes and policies, including HTTPS, cookie handling, and referrer exposure; done means the project has a complete, explicit framework for each format.