FEATURE: New plugin API to check if upload is used (#15545)

This commit introduces two new APIs for handling unused uploads, one
can be used to exclude uploads in bulk when the data model allow and
the other one excludes uploads one by one.

app/models/upload.rb

@@ -64,6 +64,30 @@ class Upload < ActiveRecord::Base
     )
   end
 
+  def self.add_unused_callback(&block)
+    (@unused_callbacks ||= []) << block
+  end
+
+  def self.unused_callbacks
+    @unused_callbacks
+  end
+
+  def self.reset_unused_callbacks
+    @unused_callbacks = []
+  end
+
+  def self.add_in_use_callback(&block)
+    (@in_use_callbacks ||= []) << block
+  end
+
+  def self.in_use_callbacks
+    @in_use_callbacks
+  end
+
+  def self.reset_in_use_callbacks
+    @in_use_callbacks = []
+  end
+
   def self.with_no_non_post_relations
     scope = self
       .joins(<<~SQL)