[PoC] XMLCast (SQL/XML X025)

Hi,

This is a PoC that implements XMLCast (SQL/XML X025), which enables
conversions between SQL and XML data type.

It basically does the following:

* When casting an XML value to a SQL data type, XML values containing
XSD literals will be converted to their equivalent SQL data type.
* When casting from a SQL data type to XML, the cast operand will be
translated to its corresponding XSD data type.

SELECT xmlcast(now() AS xml);
             xmlcast              
----------------------------------
 2024-07-02T17:03:11.189073+02:00
(1 row)

SELECT xmlcast('2024-07-02T17:03:11.189073+02:00'::xml AS timestamp with
time zone);
            xmlcast            
-------------------------------
 2024-07-02 17:03:11.189073+02
(1 row)

SELECT xmlcast('P1Y2M3DT4H5M6S'::xml AS interval);
            xmlcast            
-------------------------------
 1 year 2 mons 3 days 04:05:06
(1 row)

SELECT xmlcast('<foo&bar>'::xml AS text);
  xmlcast  
-----------
 <foo&bar>
(1 row)

SELECT xmlcast('1 year 2 months 3 days 4 hours 5 minutes 6
seconds'::interval AS xml) ;
    xmlcast     
----------------
 P1Y2M3DT4H5M6S
(1 row)

SELECT xmlcast('42.73'::xml AS numeric);
 xmlcast
---------
   42.73
(1 row)

SELECT xmlcast(42730102030405 AS xml);
    xmlcast     
----------------
 42730102030405
(1 row)

Is it starting in the right direction? Any feedback would be much
appreciated.

Best,
Jim