Implement OAuth2 authentication support for custom LiteLLM providers

- Add OAuth2Config and OAuth2ConfigLoader for litellm_config.json configuration
- Add OAuth2TokenManager for token acquisition, caching, and refresh
- Extend LLM class to support OAuth2 authentication with custom providers
- Add comprehensive tests covering OAuth2 flow and error handling
- Add documentation and usage examples
- Support Client Credentials OAuth2 flow for server-to-server authentication
- Maintain backward compatibility with existing LLM providers

Fixes #3114

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

docs/oauth2_llm_providers.md

@@ -0,0 +1,147 @@
+# OAuth2 LLM Providers
+
+CrewAI supports OAuth2 authentication for custom LiteLLM providers through configuration files.
+
+## Configuration
+
+Create a `litellm_config.json` file in your project directory:
+
+```json
+{
+  "oauth2_providers": {
+    "my_custom_provider": {
+      "client_id": "your_client_id",
+      "client_secret": "your_client_secret", 
+      "token_url": "https://your-provider.com/oauth/token",
+      "scope": "llm.read llm.write"
+    },
+    "another_provider": {
+      "client_id": "another_client_id",
+      "client_secret": "another_client_secret",
+      "token_url": "https://another-provider.com/token"
+    }
+  }
+}
+```
+
+## Usage
+
+```python
+from crewai import LLM
+
+# Initialize LLM with OAuth2 support
+llm = LLM(
+    model="my_custom_provider/my-model",
+    oauth2_config_path="./litellm_config.json"  # Optional, defaults to ./litellm_config.json
+)
+
+# Use in CrewAI
+from crewai import Agent, Task, Crew
+
+agent = Agent(
+    role="Data Analyst",
+    goal="Analyze data trends",
+    backstory="Expert in data analysis",
+    llm=llm
+)
+
+task = Task(
+    description="Analyze the latest sales data",
+    agent=agent
+)
+
+crew = Crew(agents=[agent], tasks=[task])
+result = crew.kickoff()
+```
+
+## Environment Variables
+
+You can also use environment variables in your configuration:
+
+```json
+{
+  "oauth2_providers": {
+    "my_provider": {
+      "client_id": "os.environ/MY_CLIENT_ID",
+      "client_secret": "os.environ/MY_CLIENT_SECRET",
+      "token_url": "https://my-provider.com/token"
+    }
+  }
+}
+```
+
+## Supported OAuth2 Flow
+
+Currently supports the **Client Credentials** OAuth2 flow, which is suitable for server-to-server authentication.
+
+## Token Management
+
+- Tokens are automatically cached and refreshed when they expire
+- A 60-second buffer is used before token expiration to ensure reliability
+- Failed token acquisition will raise a `RuntimeError` with details
+
+## Configuration Schema
+
+The `litellm_config.json` file should follow this schema:
+
+```json
+{
+  "oauth2_providers": {
+    "<provider_name>": {
+      "client_id": "string (required)",
+      "client_secret": "string (required)",
+      "token_url": "string (required)",
+      "scope": "string (optional)",
+      "refresh_token": "string (optional)"
+    }
+  }
+}
+```
+
+## Error Handling
+
+- If OAuth2 authentication fails, a `RuntimeError` will be raised with details
+- Invalid configuration files will raise a `ValueError` with specifics
+- Network errors during token acquisition are wrapped in `RuntimeError`
+
+## Examples
+
+### Basic OAuth2 Provider
+
+```python
+from crewai import LLM
+
+llm = LLM(
+    model="my_provider/gpt-4",
+    oauth2_config_path="./config.json"
+)
+
+response = llm.call("Hello, world!")
+print(response)
+```
+
+### Multiple Providers
+
+```json
+{
+  "oauth2_providers": {
+    "provider_a": {
+      "client_id": "client_a",
+      "client_secret": "secret_a",
+      "token_url": "https://provider-a.com/token"
+    },
+    "provider_b": {
+      "client_id": "client_b", 
+      "client_secret": "secret_b",
+      "token_url": "https://provider-b.com/oauth/token",
+      "scope": "read write"
+    }
+  }
+}
+```
+
+```python
+# Use different providers
+llm_a = LLM(model="provider_a/model-1", oauth2_config_path="./config.json")
+llm_b = LLM(model="provider_b/model-2", oauth2_config_path="./config.json")
+```