Add migration hook event

[iamacook]

Is your feature request related to a problem? Please describe.

There is no notification of newly added fields, only of updates.

Having added and expecting contractAddresses, we encountered the issue where the relevant property was not cache. This caused validation to break. We had to therefore manually clear the cache, whereas this should happen automatically

Describe the solution you'd like

When a model changes, a hook event should be emitted.

This could be done comparitively (e.g. post-migration) or (if not possible) when the service restarts.