fix: Add support for cx_Oracle's DB_TYPE_LONG_RAW

[helensilva14]

Closes #1094

Reference: https://cx-oracle.readthedocs.io/en/latest/api_manual/module.html#cx_Oracle.DB_TYPE_NCHAR

/gcbrun

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[nj1973]

I don't think adding LONG RAW to _type_mapping is enough on its own.
I suspect we also need to register the type like we do for other data types, however this is not simple because SQL Alchemy doesn't appear to have LONG RAW support either.

[nj1973]

/gcbrun

[nj1973]

The changes in this PR prevent an exception being throw when validating a table that has a LONG RAW column, they do not add functionality to validate the data in LONG RAW columns. That would be a different issue. I don't think row validation is possible on Oracle LONG [RAW] data. We might be able to make column validation work but, like I said, I think that should be a new issue.

[nehanene15]

LGTM!

[nj1973]

LGTM too!