App Configuration references in App Configuration

Currently App Configuration supports adding Azure Key Vault references, and the providers can resolve those values. Azure App Service configuration also supports references like this, but it works with both Key Vault references and App Configuration references.

We would like to have App Configuration references to other App Configuration values, so basically we would like to create aliases inside App Configuration.

The use case is that we have some configuration values that are used by multiple applications and we would like to specify these values at a single place (so if we need to change it, we don't have to find all the configs and modify all of them). An example of this could be the URL of a service endpoint that is used in multiple applications.
On the other hand, we would like to use hierarchic namespaces (or use another App Configuraton for each app) to group the configs of an app. So in the example, we would specify the URL in an app config, and in the application-specific config we would reference that one value as the single source of truth.

Example App Config reference:
```json
{
"key": "my-application:weather-api-url",
"label": null,
"value": "{\"uri\":\"https://my-app-configuration.azconfig.io/configs/weather-api-url\"}",
"content_type": "application/vnd.microsoft.appconfig.appconfigref+json;charset=utf-8",
"tags": {}
}
```

Main config value

```json
{
"key": "weather-api-url",
"label": null,
"value": "https://weather-service.com/api/",
"content_type": "",
"tags": {}
}
```

Similar Key Vault reference

```json
{
"key": "my-application:weather-api-url",
"label": null,
"value": "{\"uri\":\"https://my-keyvault.vault.azure.net/secrets/weather-api-url\"}",
"content_type": "application/vnd.microsoft.appconfig.keyvaultref+json;charset=utf-8",
"tags": {}
}
```

Compared to a Key Vault reference, the content type would use `appconfigref` instead of `keyvaultref`, and the uri in the value would point to an App Config value.