Add config file support and structure for multiple LLMs

## Migrations
- Adjusted the version specification of `serde` for consistency and clarity.
- Enhanced release profile settings for optimization.

## Dependencies
- Introduced the `dirs` and `toml` crates to facilitate reading from configuration files.
- Added development dependencies (`assert_cmd` and `predicates`) to aid in testing and command assertions.

## README
- Updated the README.md to detail reading from a configuration file and future multi-LLM support.
- Added a privacy notice for OpenAI processing only.
- Included a new "Prove It!" section for practical usage demonstration.
- Clarified installation prerequisites regarding the OpenAI API key.

## Models
### src/config.rs
- Introduced a `Config` struct consolidating configuration settings for model selection.
- Implemented `from_sources` method to read configurations from multiple sources (CLI, env vars, TOML, defaults).
- Created utility functions to find and read a TOML configuration file, enhancing user customization.
- Established groundwork for supporting multiple models in the future.

### src/llm/mod.rs
- Introduced `LlmClient` trait for a standardized LLM interface.
- Implemented `NoopClient` struct for testing purposes without a functional LLM.
- Prepared code structure for future LLM integration.

### src/llm/ollama.rs
- Introduced `ollama.rs` for future functionality related to a specific LLM.
- Organized code structure to lay the groundwork for multiple LLM support.

### src/llm/openai.rs
- Introduced `LlmClient` implementation for OpenAI, enabling requests to the OpenAI Chat Completions API.
- Implemented methods for summarizing files and generating commit/PR messages.
- Added data structures for managing API requests/responses.

## Main Application
### src/main.rs
- Introduced `config` and `llm` modules for better organization and future multi-LLM support.
- Changed struct fields and methods to public for external access.
- Updated configuration resolution to use the new `Config` module.

## Tests
### tests/cli_basics.rs
- Added `cli_basics.rs` to validate CLI behavior, ensuring correct responses to `--help` and `--version` flags.
- This supports the ability to read from a config file while ensuring proper CLI interaction.

Author: MikeGarde