Allow static web app to reference custom app settings

### Scenario

When we have different configuration setting values for apps in different environments, it would be a lot more convenient to create app settings just as we could with app services (web apps / function apps).

An example we encountered recently is during our custom OIDC authentication setup for a static web app. Per [this section in doc](https://docs.microsoft.com/en-us/azure/static-web-apps/authentication-custom?tabs=openid-connect#configure-a-custom-identity-provider), **MY_PROVIDER_CLIENT_ID** and **MY_PROVIDER_CLIENT_SECRET** are the only settings that can be used as app settings (apart from some app settings used by Managed Functions). Thus, our attempt below did not work as expected:

When we tried to have the wellKnownOpenIdConfiguration property reference an app setting named CONFIG_URL, the app seemed to simply take the string value "CONFIG_URL" as the URL instead of referencing the actual URL value in the app setting.

**In staticwebapp.config.json**:
```json
"openIdConnectConfiguration": {
...
"wellKnownOpenIdConfiguration": "CONFIG_URL",
...
}
```
**In Application Settings**:
```
CONFIG_URL https:///.well-known/openid-configuration
```

### Feature Request

We'd like to be able to set and reference app settings the same way we could in app services. In the above scenario, ideally we could reference the app setting to retrieve different URLs for apps in different environments, instead of hard-coding them with extra setups.