Support raw columns when creating virtual tables in schema_dump

Previously, ``structure_dump_virtual_column`` would not specify the required ``data_length`` parameter when  generating temporary tables.

This causes Oracle to raise ``ORA-00906: missing left parenthesis`` during ``structure_load``

This pull request adds the required parameter, resulting in well-formed creation statements.

### Example Output

#### Before
> CREATE GLOBAL TEMPORARY TABLE "RUPD$_WM_ARTKEYWORD" (
 "CHANGE_VECTOR$$" RAW
)

#### After
> CREATE GLOBAL TEMPORARY TABLE "RUPD$_WM_ARTKEYWORD" (
 "CHANGE_VECTOR$$" RAW(255)
)