Revert "Remove deprecated Transaction#store"

This reverts commit 4a4cb092929e242b4530a2e6940a48c7610b7a98, as
c85f39e
was removed from the project tree.

lib/appsignal/transaction.rb

@@ -80,6 +80,7 @@ def initialize(transaction_id, namespace, request, options = {})
       @paused = false
       @discarded = false
       @tags = {}
+      @store = Hash.new({})
       @options = options
       @options[:params_method] ||= :params
 
@@ -130,6 +131,10 @@ def discarded?
       @discarded == true
     end
 
+    def store(key)
+      @store[key]
+    end
+
     def params
       return @params if defined?(@params)
       request_params