Support environment variable placeholders in configuration files

## Summary

Dev Proxy config files (devproxyrc.json and plugin config files like CrudApiPlugin API files) don't support environment variable placeholders. This means values that vary per environment — like Entra app client IDs or tenant IDs — must be hardcoded in config files, or users need to write scripts to patch them before running Dev Proxy.

## Example: the workaround today

In the [da-ristorante-api-devproxy-entra](https://github.com/pnp/copilot-pro-dev-samples/tree/main/samples/da-ristorante-api-devproxy-entra) sample, we need to inject `ENTRA_APP_CLIENT_ID` and `ENTRA_APP_TENANT_ID` into the CrudApiPlugin API files. Because Dev Proxy doesn't support this, the sample includes a [Node.js script](https://github.com/pnp/copilot-pro-dev-samples/blob/main/samples/da-ristorante-api-devproxy-entra/scripts/update-devproxy-config.js) that reads `.env.local` and rewrites the JSON config files before each run.

The config file looks like this:

```json
{
"entraAuthConfig": {
"audience": "",
"issuer": "https://login.microsoftonline.com//v2.0"
}
}
```

And the script manually replaces those placeholders with values from env files. This works but adds friction — extra tooling, an extra build step, and mutated config files that can accidentally get committed.

## What I'd like to see

Support for environment variable references in config files, for example:

```json
{
"entraAuthConfig": {
"audience": "${ENTRA_APP_CLIENT_ID}",
"issuer": "https://login.microsoftonline.com/${ENTRA_APP_TENANT_ID}/v2.0"
}
}
```

When Dev Proxy loads a config file, it would resolve `${VAR_NAME}` placeholders against environment variables (or the `env` config section).

## Why this seems feasible

Dev Proxy already has closely related infrastructure:

- **Path tokens**: `~appFolder` and `~dataFolder` are resolved via `ProxyUtils.ReplacePathTokens` when loading config/plugin paths
- **`@dynamic` tokens**: used in mock responses and rate limiting headers, resolved at runtime
- **`ProxyUtils.ReplaceVariables`**: a general-purpose string replacement utility that replaces variable references in a string given a dictionary of values
- **`IProxyConfiguration.Env`**: the proxy configuration already exposes a `Dictionary Env` property
- **`MinimalPermissionsPlugin`** already uses `ProxyUtils.ReplaceVariables(fileContents, ProxyConfiguration.Env, v => $"{{{v}}}")` to resolve `{VAR}` placeholders in OpenAPI spec files

The pattern is already proven in plugins — it just needs to be applied when loading configuration files too.

## Benefits

- No more wrapper scripts to inject environment-specific values
- Config files stay clean and committable (no secrets, no environment-specific values)
- Consistent with how other tools handle this (Docker Compose, Azure Pipelines, GitHub Actions, etc.)
- Works naturally with `.env` files and CI/CD environments

Empieza rastreando la carga de configuración para devproxyrc.json y los archivos de configuración de los plugins; después, lee ProxyUtils.ReplaceVariables, IProxyConfiguration.Env y la ruta de reemplazo existente de MinimalPermissionsPlugin. Verifica cómo se representan los valores de entorno y la sección de configuración env antes de decidir dónde debe realizarse la resolución. La tarea está terminada cuando los marcadores de posición ${VAR_NAME} se resuelven de forma coherente en los archivos de configuración mostrados sin requerir un script de reescritura, con cobertura para marcadores de posición incrustados como la URL del issuer.

Escrito por el modelo de indexación a partir del texto del issue.