Support Snapshot in RelMdColumnOrigins

Currently `RelMdColumnOrigins` does not define a specialized method for `Snapshot` node. Thus, a request for origin column on a temporal query will always return NULL.

For example:

select productid from products_temporal
for system_time as of timestamp '2011-01-02 00:00:00' 

The origin column of `productid` is NULL.

We could simply propagate the call to its children like many other RelNode types do.