Add a Conversational ReAct Agent

[nsarrazin]

This new HfChatAgent can:

• Decide whether to use tools or not
• Explicitly plan ahead before acting (plan-and-execute capability)
• Use the input of previous tools when deciding the next step (this makes web search as a tool possible, wasn't possible with transformers agents style agents, because all steps are executed at once)
• Formulate a final answer based on the outputs of tools in scratchpad
• Work conversationally, can do follow-up things in a conversation

Things that I'm still fixing:

• Passing multimodal outputs from one tool as an input to the next tool
• Implementing web search as a tool (very simple, just copy the code from chat-ui for now)
• Make multimodal inputs work on node.js side (this breaks sveltekit integration)
• Test the prompts a bit more to make sure they're reliable
• Finish an updated demo that showcases the scratchpad and which tools are being called when. (simple, just adapt the current poc-agents.js)

If I have time I also want to refactor a bunch of things I'm not happy with in the HfChatAgent, this is not ready for review atm, code still need to be cleaned up. Done!