feat: Add support for Gemini completion and embedding models

[cvauclair]

Feature Request

Add Gemini as a new model provider for Rig.

Motivation

Gemini is a popular model provider that Rig should support and they have a REST API which should be relatively easy to integrate.

Proposal

Create a new module rig::providers::gemini which implements completion and embedding model clients for Google's Gemini.

Use the existing provider integrations as a guide (see rig::providers::openai and rig::providers::cohere).

You can find a collection of examples using the REST API (implemented in Python) here