[Migration] Enhance schema definition for diffToApply

[mdartic]

In migration.class.ts there is an issue when setting the diffToApply property of the migration.

There is a conflict between the schema for the creation of a migration (that don't have the diffToApply)
and the need of this property before injecting it in database.

https://github.com/locokit/locokit/pull/237/files/8712f5dd5454cadb1ff776dbd5b0ae47d0f17a7f#diff-825d9cd9bd27dceb54f741eed814e8488f513e1b54ffec28669de25838eb224cR440-R443

This issue is here to fix this issue, by better understanding how to use schemas & feathers.
There is surely sthg to do with external schema (the API contract, user don't have to give the diffToApply property)
versus internal schema, where diffToApply is required before injection.