[BFCL] More flexibly run any model, without a model config present.

[aaronvg]

Describe the issue
Every new model release you have to add a new model config. This is time consuming and tedious as the configs people add in their local branches may conflict with the ones the github maintainers add later on.

Proposed Changes
Let users specify the provider and model automatically via the CLI, without having to define it in a config.

Assuming you already support the Fireworks provider, I should be able to run

uv run bfcl generate --provider fireworks --model moonshot-ai/kimi-k2 --tool-format FC