Fix: Persist service credentials during silent install (JENKINS-71643)

[viru0909-dev]

Fixes #703

The Issue
During a silent installation (e.g., msiexec /i jenkins.msi /quiet), the SERVICE_USERNAME and SERVICE_PASSWORD properties were being stripped when the Windows Installer switched from the client process (UI sequence) to the elevated system process (Execute sequence). This caused the service to fail to start or default to LocalSystem.

The Fix
SERVICE_USERNAME: Added the Secure="yes" attribute to the property definition.

SERVICE_PASSWORD: Used SetProperty elements (scheduled After="AppSearch") to dynamically add this property to the SecureCustomProperties and MsiHiddenProperties lists at runtime.

Note: This approach was chosen over adding attributes directly to SERVICE_PASSWORD because that property is defined in an imported WiX library, which caused "Duplicate Symbol" errors during compilation when redefined.

Verification
Verified on Windows 10.

I built the MSI locally from this branch and performed a silent installation with verbose logging: msiexec /i "bin\Release\en-US\jenkins-2.544.msi" /quiet /lv! install.log SERVICE_USERNAME="TestUser" SERVICE_PASSWORD="Password123"

Result:

The log confirms SERVICE_USERNAME was successfully passed to the server process.

The log confirms SERVICE_PASSWORD was successfully masked (**********).

[viru0909-dev]

@MarkEWaite I have verified the fix on a Windows machine.

Verification Results: I built the MSI locally and ran a silent install:
msiexec /i "bin\Release\en-US\jenkins-2.544.msi" /quiet /lv! install.log SERVICE_USERNAME="TestUser" SERVICE_PASSWORD="Password123"

As shown in the attached screenshot of the verbose log, the SERVICE_USERNAME is successfully passed to the Server (s) execution sequence, and the SERVICE_PASSWORD is correctly masked.

**

I also updated the implementation to use SetProperty elements scheduled After="AppSearch", which avoids the "Duplicate Symbol" error I encountered with the previous approach.