NEW Add extension point to DataObject->hydrate()

[madmatt]

This PR creates a new extension point in DataObject->hydrate(). For reference, hydrate() is called on every newly created DataObject when data is retrieved from the database (e.g. during iteration of a DataList or similar).

This extension allows you to override hydrated columns, or pre-fetch some lazy-loaded ones. In my case, I am upgrading the madmatt/silverstripe-encrypt-at-rest module to CMS4 and needed a way to inject into the hydration process (as CMS3 hydrated with DBField objects but CMS4 just hydrates plain old scalar values - which in my case is an encrypted string).

[emteknetnz]

Approach seems fine. Would you be able to write a unit test for this?

[madmatt]

Hey @emteknetnz , I've added a test to DataObjectTest and confirmed that phpunit has passed that new test as well as all other tests on DataObjectTest. It appears that other PRs are failing on similar tests to what this one is, so I presume these are just broken tests that need to be fixed separately.

[emteknetnz]

Looks great. Travis failures are unrelated