Add authentication/authorization for creating new entries

Not everyone should be able to create new entries.
Add permission for this, so only persons with a specific token (OAuth2 JWT bearer token?) can create new entries using the Functions.

Start by locating the Azure Functions entry point that creates new entries and read how those requests are currently handled. Clarify the required token and authorization behavior before implementation. Done means unauthenticated or unauthorized requests cannot create entries, while requests with the agreed credentials can.

Written by the indexing model from the issue text.