Add additional OWNER for feature_engagement client usage

The feature_engagement component has a public API that requires clients
to add some configuration and boilerplate code within the component
itself, such as the base::Feature, and chrome://flags configuration.

This leads to clients having to add an additional reviewer without
necessarily adding value to the review process. twellington@ has for a
long time reviewed many such changes, but has lacked the ability to sign
off on the feature_engagement parts, even though they are well aware of
the requirements and typical usage patterns.

This CL therefore adds twellington@ as an OWNER for the relevant files
that clients typically have to edit.

Bug: None
Change-Id: Ibabef70bbe21c089d9a3a6043d2362edc7fb20ef
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642745Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666079}

components/feature_engagement/OWNERS

 dtrainor@chromium.org
 nyquist@chromium.org
 
+per-file *event_constants.*=twellington@chromium.org
+per-file *feature_configurations.cc=twellington@chromium.org
+per-file *feature_constants.*=twellington@chromium.org
+per-file *feature_list.*=twellington@chromium.org
+per-file *EventConstants.java=twellington@chromium.org
+per-file *FeatureConstants.java=twellington@chromium.org
+
 # COMPONENT: Internals>FeatureEngagement