Support configuring model

[jerop]

TL;DR

Users should be able to choose with model to use in their workflows

Detailed design

Gemini CLI sets this using GEMINI_MODEL environment variable: https://github.com/google-gemini/gemini-cli/blob/main/docs/cli/configuration.md#environment-variables--env-files

Additional information

No response

[jerop]

cc @vivekkairi