gptel supports tool-use (function calling), help me test it!

[karthink]

I've added tool-use/function calling support to all major backends in gptel -- OpenAI-compatible, Claude, Ollama and Gemini.

screencast_20241222T075329.mp4

I'm looking for folks willing to give it a spin, kick the tires a bit. Details and instructions in #514 -- please use this issue for reporting issues.