Migration to plz for requests, model context size updates for OpenAI and Gemini

• Move to plz backend, which uses curl. This helps move this package to a stronger foundation backed by parsing to spec. Thanks to @r0man for contributing the plz extensions that enable this, which are currently bundled in this package but will eventually become their own separate package.
• Add model context information for Open AI's GPT 4-o.
• Add model context information for Gemini's 1.5 models.