Convert `event_stream.application.main` into an `App` object

Many similar applications have the user define something like:

```python
app = ExampleApplication(**options_from_somewhere)

@app.register
def some_function(...):
pass

@app.register
def other_function(...):
pass
```

In order to simplify interaction and make it a little more extensible and less reliant on complex configurations.

_This_ application should follow a similar model and have the ability to create one of these `app` objects that things may attached to. The current `main` functionality does not necessarily need to be removed.

Start by reading the current event_stream.application.main functionality and identify how configuration and registration currently work. Define the App object and its registration interface so users can attach functions as shown, while preserving main if appropriate; done means the application supports this object-based interaction and remains usable.

Written by the indexing model from the issue text.