Add GitLab merge request support

[DanielAlonso97]

Summary

• add a GitLab REST provider for merge request discovery and impact analysis
• select GitHub or GitLab automatically from repository configuration
• support provider-native PR/MR formatting and submodule diff expansion
• add an opt-in, in-memory Git Credential Manager fallback
• document GitLab environment variables and add focused test coverage

Validation

• 195 relevant tests passed
• git diff --check passed

The implementation keeps GitHub behavior intact and uses generic configuration only; no project-specific hosts or credentials are included.