[FEATURE] Add Windows Dialog Support for Serverless

Is your feature request related to a problem?

Support for OpenSearch Serverless was added in #70. This did not including updating the Windows Dialog GUI to support the new ServerlessOverride configuration option. While this is not strictly necessary, it would improve the user experience.

What solution would you like?

I have no experience in this domain, but it seems like we need to:

• Update resource.h to add an IDC_SERVERLESS_OVERRIDE constant
• Update opensearch_odbc.rc to add an "Serverless Override" button to the Windows dialog.
• Update dconn_FDriverConnectProc in drvconn.c to correctly set focus for serverless override.
• Update dlg_wingui.c: SetDlgStuff and GetDlgStuff to populate dialog from the ConnInfo configuration struct and populate the ConnInfo struct from the dialog, respectively.

What alternatives have you considered?

None

Do you have any additional context?

None

Start with resource.h and opensearch_odbc.rc to see how existing Windows dialog controls are declared and laid out. Trace dconn_FDriverConnectProc and the SetDlgStuff/GetDlgStuff functions in dlg_wingui.c, then wire the ServerlessOverride value through the dialog and ConnInfo structure. Done means the Windows dialog exposes and preserves the Serverless Override setting with correct focus behavior.