feat: add a universal API for reasoning

- [x] I have looked for existing issues (including closed) about this

## Feature Request
Allow reasoning to be turned on and configured through provider-agnostic `AgentBuilder` methods.

### Motivation
Currently, enabling reasoning in rig is very provider-specfic with configuration usually being done through the miscellaneous additional_params method. This makes writing multi-provider AI software more difficult as the user of the library must configure each provider seperately.

### Proposal
This feature might be a little hard to implement as almost all providers have very different configuration methods for reasoning, with some providers not supporting it at all. However, I feel like most configuration should be abstractable into generic methods, reserving untranslatable provider-exclusive options for additional_params (e.g. Gemini's reasoning budget).

Look at the AgentBuilder struct and existing provider-specific reasoning configurations. Examine how additional_params is currently used. The goal is to design a unified API that abstracts reasoning configuration across providers, handling provider-specific options via a fallback mechanism. Start by reviewing the provider modules to understand their differing approaches.

索引モデルが issue の本文から書いたものです。