[CVaR]: Add VaR threshold variable

[datejada]

Description

• Create the variable VaR threshold variable ($\mu$) at confidence level $\alpha$. Use a descriptive name value_at_risk_threshold_mu
  • Add a new file for this variable here https://github.com/TulipaEnergy/TulipaEnergyModel.jl/tree/main/src/variables
  • Double-check if we actually need to add the corresponding SQL query: https://github.com/TulipaEnergy/TulipaEnergyModel.jl/blob/main/src/sql/create-variables.sql
  • Only create the new variable if the value of the $\lambda$ parameter (see [CVaR]: Add new model parameter for risk aversion #1524) is greater than zero
• The variable is continuous and positive (double check for upper bound)
• It doesn't depend on any index, and the units are the currency of the objective function
• Add a test that checks that the variable is created when the $\lambda$ parameter is greater than zero