The problem in the bug is that the ML backend can return Yelp ML suggestions
even when `suggest.yelp` is false. The Yelp feature also doesn't discard them
when the pref is false.
This patch adds an `isMlIntentEnabled` getter to `BaseFeature`. It also adds
`mlIntent`, which the Yelp feature already defined but I forgot to add and
document in `BaseFeature`. So now `BaseFeature` and `SuggestBackendMl` handle
enabled/disabled suggestion types very similarly to the Rust backend.
Depends on D229050
Differential Revision: https://phabricator.services.mozilla.com/D229083
03b375ad23