Adds image expir trigger configuration field (quay#1364)

Co-authored-by: Steven Smith <[email protected]>

Author: stevsmit

modules/config-fields-tag-expiration.adoc

@@ -49,6 +49,10 @@ Can be set to seconds (`s`), days (`d`), months (`m`), weeks (`w`), or years (`y
 
 |*AUTO_PRUNING_DEFAULT_POLICY_POLL_PERIOD* |Integer | The period in which the auto-pruner worker runs at the registry level. By default, it is set to run one time per day (one time per 24 hours). Value must be in seconds.
 
+|*FEATURE_IMAGE_EXPIRY_TRIGGER* |Boolean |Allows users to set up notifications on image expiration. + 
+ + 
+**Default:** False
+
 |===
 
 .Example tag expiration example YAML
@@ -69,6 +73,7 @@ DEFAULT_NAMESPACE_AUTOPRUNE_POLICY:
   method: number_of_tags
   value: 10 <1>
 AUTO_PRUNING_DEFAULT_POLICY_POLL_PERIOD: 86400
+FEATURE_IMAGE_EXPIRY_TRIGGER: false
 # ...
 ----
 <1> Specifies ten tags to remain.