Go: Implement adbc.GetSetOptions on the Go ADBC drivermgr.cnxn connection type

Problem:
Calling cnxn.(adbc.GetSetOptions).SetOption(adbc.OptionKeyCurrentCatalog, "random_data") panics because the drivermgr.cnxn type doesn’t implement the full adbc.GetSetOptions interface (missing GetOption).

Solution:
Add the missing GetOption method (and any other required stubs) to the Go driver’s cnxn type so it satisfies adbc.GetSetOptions, allowing safe use of SetOption and GetOption without panics.

Inspect the Go drivermgr.cnxn connection type and its existing option methods. Confirm the requirements of adbc.GetSetOptions, then implement the missing GetOption and any other required stubs. Done means cnxn satisfies the interface and SetOption and GetOption can be called without the reported panic.

Written by the indexing model from the issue text.