Implement autocommit isolation level for cx_oracle

Implement autocommit isolation level for cx_oracle

As part of this change Oracle also gets the concept of a
default isolation level, however since Oracle does not provide a
fixed method of knowing what the isolation level would be without a
server side transaction actually in progress, for now we hardcode
just to "READ COMMITTED".

Enhanced the test suite for isolation level testing in the dialect
test suite and added features to requirements so that the supported
isolation levels can be reported generically for dialects.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #5200
Change-Id: I2c4d49da9ff80ccc228c21e196ec9a961de53478