[PATCH] Add pg_get_type_ddl() to retrieve the CREATE TYPE statement

Hello Hackers, 

I am submitting a patch as part of the Retail DDL functions project described here [1]. This patch creates a function called pg_get_type_ddl designed to retrieve the DDL statement for CREATE TYPE. Users can get the DDL by providing a TYPE name like the following for the ENUM type:

   SELECT pg_get_type_ddl('type_name_enum');

which gives you:

                      pg_get_type_ddl                      
------------------------------------------------------------
 CREATE TYPE public.type_name_enum AS ENUM ('one', 'two', 'three');

I split up the types range, enum, shell, composite, and a base type into different functions which support `pg_get_type_ddl`. Otherwise, the function seemed too long. 

1. https://www.postgresql.org/message-id/945db7c5-be75-45bf-b55b-cb1e56f2e3e9%40dunslane.net