[enhancement] Provide information about the different models

[benthamite]

In my personal config I have some extensions for gptel, including a function and associated variable that displays information about the available models:

You can find it here. Would you be interested in a pull request? (I would adapt some of the descriptions to make them less subjective and personal.)

[karthink]

Wow this looks great! Please give me a few days to take a look at your code, it's been a rough week. This ties in with some refactoring of the backend definitions I've been meaning to do that should make attaching metadata to the models easier.