Need support for ITEM data types for WFS server

[dmorissette]

As part of the WFS enhancements, we need to be able to return proper attribute
data type information in DescribeFeatureType responses.  The current version of
MapServer treats all attributes (items) as strings and this bug is to discuss
any implications/objections of a change to support attribute data type
information in MapServer 4.1.


Here is what we propose:

1- Add a mechanism in MapServer to maintain field (item) name, data type, size,
and precision.  This would likely be a new itemDefnObj that would be set by
msLayerGetItems() instead of the array of field names that it currently produces.

2- Each data source driver would then have to be modified to return proper type
information.  For a start we should make all drivers return item data types as
"string" by default, and then we should file bugs for each driver maintainer to
update their driver when they have time.

3- For the time being, attribute values would still be carried as strings
internally.  We could switch to using a union similar to OGR's OGRField later
on, but for now we'll carry all values as strings internally to limit the amount
of changes to the code.

4- The WFS DescribeFeatureType will need to be extended to publish data types. 
Note that this implies that some rules related to those data types in XML will
have to be followed.

Any comments, objections, suggestions?