feat: add region support for API base URLs

[cvejk]

Description

Adds Region support to the Contentful client. A new SetRegion setter method allows switching all API base URLs to the corresponding regional endpoint (e.g. EU) without touching constructor signatures or breaking any existing code.

Type of Change

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

Changes

• Added Region string type with RegionUS = "us" and RegionEU = "eu" constants
• Added SetRegion(region Region) *Contentful method on the client — rewrites BaseURL and UploadURL (CMA only) to the regional subdomain via a single strings.Replace
• SetRegion("") and SetRegion(RegionUS) are strict no-ops — the client returns unchanged, all existing call sites are unaffected
• Method is chainable: contentful.NewCDA(key).SetRegion(RegionEU)
• Added TestSetRegion covering all four endpoints (CMA, CMA upload, CDA, CPA), the no-op guarantee, and chainability

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.

[cvidmar]

LGTM