Where to store library specific context data?

Currently we store hapi events under `context.custom`. This location is only supposed to be used for storing user provided data. The agent is not supposed to stored data here automatically:

https://github.com/elastic/apm-agent-nodejs/blob/a077ef5462153f8bffd070b8fc41215754b6590d/lib/instrumentation/modules/hapi.js#L28

Instead we should consider something like this:

```js
context.library.hapi.event = event
```

That however would mean custom logic in our Kibana plugin.

Related: elastic/apm-agent-nodejs#12