Add Demo-API Example Showcasing Global Variables

[camilamaia]

Description:

The examples repository does not yet contain an example of the demo-api using Global Variables. Adding this will provide users with a ready-to-use reference that complements the documentation and tutorial.

This issue aims to:

• Add a new example based on the demo-api that demonstrates Global Variables in action.
• Showcase a realistic use case (e.g., base URL, authentication token, or shared headers).
• Ensure consistency with the ScanAPI documentation and tutorial.

Dependency: Blocked by demo-api#98

Reference: Feature implemented in ScanAPI #265