Added extension exchange relation

Necessary to provide low level database access to extension exchanges.

Issue: 261
Milestone: minor

app/relations/extension.rb

@@ -10,6 +10,7 @@ module Terminus
           has_many :devices, through: :extension_device, relation: :device, as: :devices
           has_many :extension_models, relation: :extension_model
           has_many :models, through: :extension_model, relation: :model, as: :models
+          has_many :extension_exchanges, relation: :extension_exchange
         end
       end
     end

app/relations/extension_exchange.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: true
+
+module Terminus
+  module Relations
+    # The extension exchange relation.
+    class ExtensionExchange < DB::Relation
+      schema :extension_exchange, infer: true do
+        associations { belongs_to :extension, relation: :extension }
+      end
+    end
+  end
+end