Table function for XML

[beikov]

Building upon #5901 and #5898, the idea is to introduce a xmltable table function that allows mapping XML as anonymous tuple.
The standard SQL syntax that I would like to adopt is nicely illustrated here: https://docs.oracle.com/database/121/SQLRF/functions269.htm#SQLRF06232
A rough ANTLR grammar for this:

xmltable : XMLTABLE "(" (xmlnamespaces ",")? xquery_string=string_literal xmltableoptions ")";
xmlnamespaces : XMLNAMESPACES "(" xmlnamespace ("," xmlnamespace)* ")";
xmlnamespace 
	: namespace=string_literal AS alias=identifier
	| DEFAULT namespace=string_literal
	;
xmltableoptions : PASSING xmlvalue=expression COLUMNS xmltable_column ("," xmltable_column)*;
xmltable_column 
	: xmltable_value_column
	| xmltable_ordinality
	;

xmltable_value_column : column_name=identifier castTargetType PATH json_path=string_literal (DEFAULT expression)? (NOT? NULL)?;
xmltable_ordinality : column_name=identifier FOR ORDINALITY;