Support git_config parameter in SourceCode or ModelTrainer

**Describe the feature you'd like**
Add `git_config` parameter support to the SourceCode class (or directly to ModelTrainer) to enable fetching source code directly from Git repositories, similar to the functionality available in JumpStart models and v2 [Estimator classes](https://sagemaker.readthedocs.io/en/v2/api/training/estimators.html).

The `git_config` parameter should accept a dictionary with the following keys:
- `repo` (required): Git repository URL (https, http, git@, or ssh://)
- `branch` (optional): Branch name (defaults to 'master')
- `commit` (optional): Specific commit hash
- `2FA_enabled` (optional): Boolean for GitHub 2FA authentication
- `username`, `password`, `token` (optional): Authentication credentials

**How would this feature be used? Please describe.**
This feature would allow users to reference training code stored in Git repositories without manually cloning them first. This is particularly useful for:

1. CI/CD pipelines - Automatically pull the latest training code from a repository
2. Team collaboration - Share training scripts via version control without S3 uploads
3. Reproducibility - Pin to specific commits for exact code versioning

**Describe alternatives you've considered**
Current workarounds include:
1. Manual cloning - Clone the repository locally before creating ModelTrainer, then use local `source_dir`
2. S3 upload - Upload code to S3 and reference it via S3 URI in `source_dir`
3. Use legacy estimators - Switch to older Estimator classes that support `git_config`

**Additional context**
The SDK already has the infrastructure for this feature:
- `sagemaker.core.git_utils.git_clone_repo()` handles Git cloning with authentication
- `sagemaker.core.git_utils._sanitize_git_url()` provides security validation
- JumpStart models (`JumpStartModelInitKwargs`) already support `git_config`

Commencez par SourceCode et ModelTrainer, puis comparez leurs chemins d’initialisation avec JumpStartModelInitKwargs et le comportement de git_config de v2 Estimator. Lisez d’abord sagemaker.core.git_utils.git_clone_repo() et _sanitize_git_url() ; le travail est considéré comme terminé lorsque les clés de dictionnaire spécifiées sont acceptées et que le dépôt, la branche ou le commit demandé est récupéré avec une authentification prise en charge.

Rédigé par le modèle d'indexation à partir du texte de l'issue.