refactor: Update variableService to be more maintenable

[HugoRCD]

Description

Enhance the maintainability of variableService.ts by implementing the following improvements:

• Simplified variable creation, whether in batch or individual instances
• A rewritten overwrite/merge method that is more readable and easier to maintain
• Preparation for future updates, such as issues feat: add support for env var variables #97 and feat: Shared env variables (across several project) #43

The primary objective of this issue is to simplify every variable service and function, ensuring they are well-documented and easy to maintain.

Additional context

No response