Converted SQL mail into multiple files. (apache#1401)

Converted SQL mail into multiple files.

This pull request is aimed at improving the overall quality of our SQL codebase
by converting the main SQL (age-x.x.x.sql) file into multiple smaller files in the
sql directory. The primary goal is to enhance code readability, maintainability,
and organization for the benefit of the development team.

(cherry picked from commit f6e6af7)

sql/age_scalar.sql

@@ -157,14 +157,6 @@ RETURNS NULL ON NULL INPUT
 PARALLEL SAFE
 AS 'MODULE_PATHNAME';
 
-CREATE FUNCTION ag_catalog.age_tostringlist(variadic "any")
-    RETURNS agtype
-    LANGUAGE c
-    IMMUTABLE
-RETURNS NULL ON NULL INPUT
-PARALLEL SAFE
-AS 'MODULE_PATHNAME';
-
 CREATE FUNCTION ag_catalog.age_size(variadic "any")
     RETURNS agtype
     LANGUAGE c