feat: add region support to config, generator, templates and testapi

[cvejk]

Description

Adds support for Contentful's EU infrastructure region. The region can be specified via the -region flag or the region field in the config YAML. When set, the generated client code calls .SetRegion() on all client modes (CDA, CPA, CMA) as well as the offline client.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📝 Documentation
• ♻️ Refactoring
• ⚡ Performance
• ✅ Tests
• 🔧 Build/CI

Changes

• Added Region field to Config struct
• Added -region CLI flag to main.go
• GenerateAPI now validates the region via contentful.ParseRegion and passes it to the template
• Template (contentful_vo_lib.gotmpl) conditionally emits .SetRegion() in both getContentfulAPIClient and NewOfflineContentfulClient
• Added generator tests covering EU region, empty region, and invalid region inputs
• Regenerated test/testapi to reflect the refactored getContentfulAPIClient function

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.

Notes

This PR depends on foomo/contentful#20 which adds the SetRegion/ParseRegion API. Once that PR is merged and a new release is cut, the contentful dependency version in go.mod should be bumped accordingly.

[cvidmar]

LGTM