Microsoft Entra ID authentication in Azure Functions (all languages, without EasyAuth)

## Motivation

Today, securing Azure Functions with Microsoft Entra ID typically requires one of two approaches:

### EasyAuth (App Service Authentication)

Simple to enable, but:

- Limited per‑function control
- Harder to test locally
- Awkward for advanced scenarios (multi‑tenant, custom claims, mixed auth)

### Custom JWT validation in code

Flexible, but:

- Boilerplate repeated across functions and repos
- Inconsistent patterns between languages
- Easy to misconfigure (issuer, audience, JWKS, clock skew, etc.)