Add an API Endpoint Node

Chainforge should add an **API Endpoint Node** to ChainForge. This will allow users to prompt engineer and systematically battletest prompts against their own LLM-integrated systems via API calls instead of a LLM provider like OpenAI, and Claude.

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**Potential Capabilities:**
- Send prompts directly to external APIs.
- Evaluate API responses within ChainForge workflows.
- Enable automated testing flows callable via the ChainForge CLI