[C++][FlightRPC][ODBC] Add custom known field on ODBC Windows DSN window doesn't work properly #55603

Describe the bug, including details regarding any error messages, version, and platform.

There are 2 bugs:

1. [Affects Windows only] DSN window issue - Adding a StringColumnLength property when configuring the Apache Arrow Flight SQL DSN in ODBC Data Source Administrator does not work. When user adds StringColumnLength property and then close and re-open the DSN window, the field is gone.
2. [Affects all 3 platforms] Even when StringColumnLength is set in DSN registry, the ODBC driver cannot read it

StringColumnLength is not the only case, some other known keys are affected too.

Component(s)

FlightRPC, C++