Add API datetime/format matcher for serialization specs, reduce factories (#33325)

spec/support/matchers/api_datetime_format.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+RSpec::Matchers.define :match_api_datetime_format do
+  match(notify_expectation_failures: true) do |value|
+    expect { DateTime.rfc3339(value) }
+      .to_not raise_error
+  end
+end