Table function for JSON

[beikov]

Building upon #5900 and #5898, the idea is to introduce a json_table table function that allows mapping JSON as anonymous tuple.
The standard SQL syntax that I would like to adopt is nicely illustrated here: https://docs.oracle.com/database/121/SQLRF/functions092.htm#SQLRF56973
A rough ANTLR grammar for this:

json_table : JSON_TABLE "(" json_expression=expression "," json_path=string_literal json_colums ")";
json_columns : COLUMNS "(" json_column ("," json_column)* ")";
json_column 
	: json_exists_column
	| json_query_column
	| json_value_column
	| json_nested_path
	| json_ordinality
	;

json_exists_column : column_name=identifier castTargetType EXISTS PATH json_path=string_literal;
json_query_column : column_name=identifier castTargetType PATH json_path=string_literal;
json_value_column : column_name=identifier castTargetType PATH json_path=string_literal;
json_nested_path : NESTED PATH json_path=string_literal json_columns;
json_ordinality : column_name=identifier FOR ORDINALITY;