[API] Adds ES_SPECIFICATION_COMMIT

The code is automatically generated from elasticsearch-specification:
https://github.com/elastic/elasticsearch-specification
The value Elasticsearch::ES_SPECIFICATION_COMMIT will be updated with the commit hash of
elasticsearch-specification in which the code is based.

Instead of writing the commit hash in each generated file, it will be saved in this constant.

Author: picandocodigo

elasticsearch-api/lib/elasticsearch/api/version.rb

@@ -18,5 +18,6 @@
 module Elasticsearch
   module API
     VERSION = '9.0.0'.freeze
+    ES_SPECIFICATION_COMMIT = 'dbb89db180052524cab77d324be4f0c00972807a'.freeze
   end
 end