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ARROW-14903: [C++] Enable CSV Writer to control string to be used for missing data

This adds the option to configure the string that is rendered in the CSV output for null values.
If this null string has any quotes, they are escaped. I decided not to deal with the fact that the user can provide a string that cause rendered values to become ambiguous. E.g. when the user supplies `42` and an integer column has a null, it will render as `42` making it indistinguishable from the actual value of 42.
The reason is that if we were to quote a valid value or the null value, this may annoy downstream applications that perform e.g. type inference for said column. But the most major drawback is that we'd need to check every valid value for equality to the configured null value, which I think may be a price too costly to pay for users making awkward choices for their null value.

Closes #11836 from johanpel/ARROW-14903

Authored-by: Johan Peltenburg <johan.peltenburg+code@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>