feat: client credentials grant for admin API

[freb97]

🔗 Linked issue

resolves #217

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Adds support for the new client credentials grant.

Admin API clients should be able to set clientId and clientSecret instead of accessToken for the admin client configuration. The access token will be obtained at runtime and stored in an unstorage mount.