fix: Enforce UTF-8 encoding in response body parsing

[akaegi]

Description

I got an issue of a wrong character encoding in the response from the userinfo endpoint when the user name contains non-ASCII characters.
This stems from the fact that http library falls back to latin encoding (http-1.2.0/lib/src/utils.dart:encodingForCharset) when the Content-Type header in the userinfo response only says application-json instead of application-json; charset=utf-8. However utf-8 is the default for application/json so oidc library should handle this gracefully ;-)

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[akaegi]

@ahmednfwela Could you check why the pipeline is "stuck" for all PRs?