[franklinkim]

Description

Allow overriding GTM tag templates with custom template files per provider via templates config sections.

Type of Change

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

Changes

• Add pkg/tagmanager/common/template/customtemplate.go with NewFromFile, Resolve and ResolveOrLookup helpers for loading templates from disk
• Add a templates config section to cookiebot, criteo, emarsys, facebook, googleanalytics, microsoftads, mixpanel, pinterest, tracify, umami and utm
• Wire providers to resolve a custom template file first, falling back to the embedded or manually installed template
• Fix the diff direction in UpsertCustomTemplate so remote state is the "before" side
• Remove the GIM-5006 / GIM-4989 custom modifications from the embedded Mixpanel tag template
• Add table-driven tests for the template resolution helpers
• Regenerate sesamy.schema.json and update README + provider docs
• Update docs/CONTRIBUTING.md workflow (make check) and PR guidelines

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.