[ADQL] Set a type to a query's resulting column when it is not originally a column.

This is easily possible for concatenations, string constants and User Defined
Functions having a FunctionDef. A new special datatype was needed for
numeric functions and operations: UNKNOWN_NUMERIC. This special type
can not be set with FunctionDef.parse(...) and it behaves exactly like the type
UNKNOWN, except that DBType.isNumeric() returns true (as .isUnknown()).
Thus, while writing the metadata of a result in TAP, nothing changes:
an UNKNOWN_NUMERIC type will be processed similarly as an UNKNOWN type:
to use the type returned from the database ResultSet or to set VARCHAR.
(no modification of TAP was needed for that)