Add performance test using Gatling

[icemc]

Scalameter the library used for performance testing in Agora doesn't have support for recent versions of Scala (2.13 and 3). Consider rewriting the performance tests using Gatling which has support for these versions of Scala.

[achalcipher]

TEAM: NULL

Gatling setup and migration materials are in place. Completed:

1. Build configuration
   Added Gatling SBT plugin (project/plugins.sbt):
   Version 4.3.0 (supports Scala 2.13 and 3)
   Updated build.sbt:
   Enabled GatlingPlugin for the cli module
   Added Gatling dependencies (3.10.5) compatible with Scala 2.13 and 3
2. Directory structure
   Created: modules/cli/src/test/scala/org/aossie/agora/performance/
   All Gatling simulations go here
3. Documentation and examples
   VoteCountingSimulation.scala: Example simulation with:
   Multiple scenarios (Borda, Majority, multi-method)
   Load injection patterns (ramp-up, constant load)
   Performance assertions (95th percentile checks)
   Annotations explaining concepts
   Minimal example class
   GATLING_MIGRATION_GUIDE.md: Scalameter → Gatling mapping
   GATLING_SETUP.md: Setup documentation
   README.md: Performance tests usage guide
   QUICK_START_GATLING.md: Quick reference
   Ready to use
   Run:
   sbt reloadsbt "project cli" "Gatling / test"
   All files are in place, configured, and ready. The example simulation demonstrates the Gatling DSL with annotations, and the migration guide maps Scalameter concepts to Gatling equivalents.