Feat - Add extra params support to create_experiment tool

[fsarachu]

Summary

• Adds 4 new optional parameters to create_experiment tool to support per-environment experiment configuration (Addresses Feature: Create per‑environment experiment rules #29 )
• Enables users to override hashAttribute for custom user bucketing (e.g., userId instead of default id)
• Allows custom trackingKey for analytics integration instead of auto-generated keys
• Introduces enabledEnvironments to control which environments receive experiment rules
• Adds environmentConditions for per-environment targeting conditions (e.g., {"targeting_attribute": true} for production only)

Test plan

• Run npm test - all 36 tests pass (12 new + 24 existing)
• Run npm run build - TypeScript compiles successfully
• Manual test: Create experiment with hashAttribute: "userId" and verify API payload includes it
• Manual test: Create experiment with custom trackingKey and verify it's used instead of auto-generated
• Manual test: Create experiment with enabledEnvironments: ["staging"] and verify rule only added to staging
• Manual test: Create experiment with environmentConditions: { "production": "{"targeting_attribute": true}" } and verify condition is set

[royalfig]

Hey @fsarachu! This is looking good. Lmk when you're ready for a review.