infra: Add PR triage action (#5293)

This is a bare minimal triage action that handle big categories.

In the future we could also label all services correctly but
I didn't felt this was required for a first iteration.

.github/assign/horizon.yml

@@ -0,0 +1,11 @@
+addReviewers: true
+
+reviewers:
+  - gdkchan
+  - Ack77
+  - marysaka
+  - TSRBerry
+
+filterLabels:
+  include:
+    - horizon