Bug
1858542 - Send Glean pings for Firefox Suggest impressions from Fenix.
This commit adds the following measurements:
* A new value for the `fx_suggest.ping_type` metric,
`fxsuggest-impression`.
* An `fx_suggest.position` metric to capture the position of the
Firefox Suggest search suggestion in the awesomebar, for impressions
and clicks.
* An `fx_suggest.is_clicked` metric, to capture whether an
`fxsuggest-impression` ping is for a clicked suggestion.
This commit also:
* Mounts `FxSuggestFactsMiddleware` to collect facts for impressions
and clicks.
* Dispatches `AwesomeBarAction`s from the `SearchDialogController` in
response to user interactions with the awesomebar and toolbar.