[Plugin request] Personal AI integration via Gemini, OpenAI and other APIs

### Description

A plugin to enable using Gemini API, OpenAI API, or other popular AI APIs for personal AI assistant integrations in AppFlowy. The plugin should allow users to connect their own API keys and seamlessly access AI features like chat, code generation, summarization, and task automation. Gemini API is free for some usage limits, making it accessible for students and regular users.

### Impact

- Students, researchers, and professionals who want to leverage personal AI assistants inside AppFlowy
- Anyone looking for AI-driven productivity, knowledge retrieval, writing assistance, or workflow automation
- Users who prefer controlling their AI usage and privacy by using their own API keys

### Additional Context

- Gemini API is part of Google AI and provides free-tier access for individuals
- OpenAI API is widely used for GPT models, supporting advanced AI capabilities
- Integration flexibility (choose API, easy setup)
- Example use cases: summarize meeting notes, generate markdown/docs, automate flows, etc.
- Plugin should allow configuration and switching between APIs