Add a registry for features that rely on the CH infrastructure

[yoavweiss]

We need a registry of all the features that rely on CH in order to:

• Have their values be included as valid CH tokens
• Determine if they are high or low entropy, to know if they should be included by default or require an opt-in
• Define the related feature policies for HTML

Such a registry should be separate from the infrastructure spec, but can live under the same repo, to make things simpler.

[yoavweiss]

Hmm, I got myself confused as there's already a registry. However, it'd still be a good idea to spin it to its own document

[yoavweiss]

Hmm, I got myself confused as there's already a registry. However, it'd still be a good idea to spin it to its own document

After going back and forth on this, we can probably have the registry live in the same document, while adding a note indicating that UAs are not required to implement all of the related features mentioned in it.

[yoavweiss]

Closed by #13