Support PercentChangeLoadAction in SA with Operator Strategies

[So-Fras]

Describe the current behavior

The percent change load action is not a supported OperatorStrategies action in OLF.

Describe the expected behavior

The percent change load action is a supported OperatorStrategies action in OLF.

Describe the motivation

In a security analysis context, we want to be able to modify a load (with a percentage change) using OperatorStrategies.

Extra Information

powsybl-core/#3189 is a prerequisite