Introduce NGINX_CUSTOM_LOCATION_BLOCK_BASE64

This variable is a very low-level feature. It allows to insert a location block into the Nginx configuration. The variable's value must be Base64-encoded.

Example of such a block (not yet encoded):
```
  location = /.well-known/com.apple.remotemanagement {
    default_type application/json;
    return 200 '{"Servers":[{"Version":"mdm-adde","BaseURL":"https://mdm.example.com/api/v2enrollment/enroll/adde"}]}';
    add_header Content-Type 'application/json' always;
  }
```