oci - support for extended max_string_size

[ilucena]

If user changes db parameter max_string_size=extended, the maximum size for varchar2 columns can be defined as 32767 maximum (Oracle 12.0).

Currently, on OCI driver the default string size is 4000. That valued is used to defined a column as VARCHAR2(4000) when in a ogr2ogr use case, the input data source doesn't provide the size of a string column.

We need to add the layer creation option, DEFAULT_STRING_SIZE, to define the default string size that users want.

Attempt to use a number bigger than 4000 when the server is using max_string_size=standard will raise a warning and the column will not going to be created and loaded.

Attempt to use a number bigger than 32767 when the server is using max_string_size=extended will also raise a warning and the column will not going to be created and loaded.

[ilucena]

In 40929:

Fix #7168 OCI - support for extended max_string_size