How to pass the service_tier (Flex Processing) parameter to OpenAI models?

[grade1000]

I propose adding support for the service_tier parameter in the OpenAIChat class. This would involve a few minor changes to agno/models/openai/chat.py:

1. Add service_tier to the OpenAIChat dataclass:

   @dataclass
   class OpenAIChat(Model):
       # ... other parameters
       service_tier: Optional[str] = None
       # ...

2. Include service_tier in the get_request_params method:

   def get_request_params(self, ...):
       base_params = {
           # ...
           "service_tier": self.service_tier,
           # ...
       }
       # ...

3. (Optional but good practice) Add service_tier to the to_dict method for serialization consistency.