Add base moderation to Providers

[zakiali]

Adding in moderation for user inputs.

• A Moderation trait is added to the base Provider struct along
• Update the base provider complete method to make a concurrent request to both the moderation and LLM endpoint. If moderation fails, we error out and abort the LLM completion if the request is still in flight. If moderation passes, we wait for the LLM response and continue as normal
• Only moderates on the latest User message
• Implemented moderation for the OpenAI provider using the openai's moderation endpoint, as an example