This adds a new "engagement" telemetry event for quick suggest results. The
event's object can be the following values:
* "block": The user dismissed ("blocked") the suggestion.
* "click": The user picked the suggestion.
* "help": The user picked the suggestion's help button.
* "impression_only": The user picked some other row.
For this particular bug, we're interested in recording blocks, but there's no
reason not to generalize that idea into an engagement event like this that
records all types of interactions.
Depends on D143254
Differential Revision: https://phabricator.services.mozilla.com/D143331
32e1ef3b0c