Add rampUpDuration parameter

[01epa]

Proposal:
Introduce a rampUpDuration parameter that defines the time interval over which the request rate gradually increases from zero (or a defined initial rate) to the target rate.

Benefits:

• Helps avoid sudden spikes in load that could distort performance measurements.
  

• Allows the system under test to warm up more realistically, mimicking production-like traffic patterns.
  

• Makes it easier to detect performance thresholds and degradation points as load increases progressively.
  

• Reduces the risk of false positives due to cold starts or uninitialized caches.