Add Change notification to bindle server

[simongdavies]

Bindle server should support event notification for changes, clients should be able to subscribe to an event stream, messages, webhook to get notifications about new bindles, new versions etc.

This will enable many scenarios including application updates, slack notifications etc.

[thomastaylor312]

I think this is very similar to #190 and they could likely be tackled as part of the same work. We should architect the webhooks to be generic enough to handle both cases