Fix MLFlow integration issue #3305

- Add mlflow as optional dependency in pyproject.toml
- Create MLFlow integration utilities in src/crewai/integrations/mlflow.py
- Add comprehensive tests for MLFlow integration
- Support graceful handling when MLFlow is not installed
- Enable mlflow.crewai.autolog() functionality as documented

Fixes #3305

Co-Authored-By: João <joao@crewai.com>

pyproject.toml

@@ -69,6 +69,9 @@ docling = [
 aisuite = [
     "aisuite>=0.1.10",
 ]
+mlflow = [
+    "mlflow>=2.19.0",
+]
 
 [tool.uv]
 dev-dependencies = [