feat: add base64 identifier to EnvParser

[NathaelB]

🔗 Linked issue

No existing issue

❓ Type of change

• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The objective is to retrieve a value that has been given in base64.
Using the key APP_KEY=base64:YWRvbmlzanMtcnVsZXM=, we can directly retrieve the value adonisjs-rules.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[RomainLanz]

Hey! 👋🏻

Thanks for the PR. I am not sure if we should directly convert it to string since we will primarily use this to store a binary key for encryption.

I believe we should keep it as a buffer and add some transformer to retrieve JSON, string or something else.

WDYT @thetutlage?