Litellm integration (#524)

litellm is a library that abstracts away details/differences for a lot
of model providers. Adding an extension, so that any provider can easily
be integrated.

---
[//]: # (BEGIN SAPLING FOOTER)
* #532
* __->__ #524

Author: rm-openai

Diff for: examples/model_providers/litellm_provider.py

@@ -0,0 +1,55 @@
+from __future__ import annotations
+
+import asyncio
+
+from agents import Agent, Runner, function_tool, set_tracing_disabled
+from agents.extensions.models.litellm_model import LitellmModel
+
+"""This example uses the LitellmModel directly, to hit any model provider.
+You can run it like this:
+uv run examples/model_providers/litellm_provider.py --model anthropic/claude-3-5-sonnet-20240620
+or
+uv run examples/model_providers/litellm_provider.py --model gemini/gemini-2.0-flash
+
+Find more providers here: https://docs.litellm.ai/docs/providers
+"""
+
+set_tracing_disabled(disabled=True)
+
+
+@function_tool
+def get_weather(city: str):
+    print(f"[debug] getting weather for {city}")
+    return f"The weather in {city} is sunny."
+
+
+async def main(model: str, api_key: str):
+    agent = Agent(
+        name="Assistant",
+        instructions="You only respond in haikus.",
+        model=LitellmModel(model=model, api_key=api_key),
+        tools=[get_weather],
+    )
+
+    result = await Runner.run(agent, "What's the weather in Tokyo?")
+    print(result.final_output)
+
+
+if __name__ == "__main__":
+    # First try to get model/api key from args
+    import argparse
+
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--model", type=str, required=False)
+    parser.add_argument("--api-key", type=str, required=False)
+    args = parser.parse_args()
+
+    model = args.model
+    if not model:
+        model = input("Enter a model name for Litellm: ")
+
+    api_key = args.api_key
+    if not api_key:
+        api_key = input("Enter an API key for Litellm: ")
+
+    asyncio.run(main(model, api_key))

Diff for: pyproject.toml

@@ -36,6 +36,7 @@ Repository = "https://github.com/openai/openai-agents-python"
 [project.optional-dependencies]
 voice = ["numpy>=2.2.0, <3; python_version>='3.10'", "websockets>=15.0, <16"]
 viz = ["graphviz>=0.17"]
+litellm = ["litellm>=1.65.0, <2"]
 
 [dependency-groups]
 dev = [
@@ -44,7 +45,7 @@ dev = [
     "pytest",
     "pytest-asyncio",
     "pytest-mock>=3.14.0",
-    "rich",
+    "rich>=13.1.0, <14",
     "mkdocs>=1.6.0",
     "mkdocs-material>=9.6.0",
     "mkdocstrings[python]>=0.28.0",