[NonGNU ELPA] Add package gptel

[Karthik Chikmagalur <karthikchikmagalur@gmail.com>]

I'd like to submit my package gptel to NonGNU ELPA:

https://github.com/karthink/gptel

gptel is an LLM (Large Language Model) client for Emacs that supports
most LLM providers that offer an HTTP API.  This includes open source
models running locally on the user's PC or network via Ollama,
Llama.cpp, Llamafiles and GPT4All, and access to larger models provided
by a growing number of companies.  This includes ChatGPT, Anthropic
Claude, Gemini, Kagi, Groq, Perplexity, TogetherAI and several more.

gptel tries to provide a uniform Emacs-y UI for all backends, and works
as both a chat interface (in dedicated chat buffers) and as a
helper/lookup agent in all Emacs buffers.  There is a demo showcasing
its many uses here:

https://www.youtube.com/watch?v=bsRnh_brggM

It has no external dependencies (Emacs packages or otherwise), but uses
Curl if it's available.

Karthik