EZP-21990: Implement Field Storage publish events

See https://jira.ez.no/browse/EZP-21990.

Adds Field Storage events upon content publishing (pre/post).
Required by https://jira.ez.no/browse/EZP-21213.

Should have enough coverage, but this should be tested.

# TODO
- [x] Improve coverage
- [x] Refactor events to use an optional `FieldStorage\EventAware` interface