Add automatic resolution of embedded object constraints during migration

[rorbech]

This adds configuration option to enable automatic handling of embedded object constraints during automatic migration through:

class RealmConfiguration {

    class Builder {
        /**
         * Sets the migration to handle schema updates with automatic migration of data.
         *
         * @param migration the [AutomaticSchemaMigration] instance to handle schema and data
         * migration in the event of a schema update.
         * @param resolveEmbeddedObjectConstraints a flag to indicate whether realm should resolve
         * embedded object constraints after migration. If this is `true` then all embedded objects
         * without a parent will be deleted and every embedded object with multiple references to it
         * will be duplicated so that every referencing object will hold its own copy of the
         * embedded object.
         *
         * @see RealmMigration
         * @see AutomaticSchemaMigration
         */
        public fun migration(
            migration: AutomaticSchemaMigration,
            resolveEmbeddedObjectConstraints: Boolean = false
        ): Builder =
            apply {
                this.migration = migration
                this.automaticEmbeddedObjectConstraintsResolution = resolveEmbeddedObjectConstraints
            }
    }
}

The above API was chosen in favor of adding additional properties/methods to the AutomaticSchemaMigration interface, as this would require users to instantiate it as anynomous instances and that seems very tedious compared to the standalone functional interface. Alternative it could also have been wrapped in a completely new type, but that just bloats the name space.

Closes #1464

[sipersso]

An observation here. Coming from the SwiftSDK and Java SDK. It seems as if for those SDK:s the resolveEmbeddedObjectConstraints defaults to true. Is there any reason why this shouldn't default to true for the Kotlin SDK?

[rorbech]

@sipersso The feature wasn't available in the C-API layer for core when launching and we didn't caught that as a required use case for the initial release. Now we cannot just update the default behavior. I will rethink if we can introduce this in a way that makes it the new default, e.g. new method or something like that.

[sipersso]

Ok... well as long as I can enable it I guess it isn't an issue. But people migrating from Java to Kotlin might get a nasty surprise if they are not aware of this difference in implementations.

[cmelchior]

Nice 🚀 .. I was a bit surprised to see the duplicated method, but I assume it is because we need to constrain it to the AutomaticRealmMigration interface rather than the RealmMigration interface?

[rorbech]

Nice 🚀 .. I was a bit surprised to see the duplicated method, but I assume it is because we need to constrain it to the AutomaticRealmMigration interface rather than the RealmMigration interface?

Yes. The option will probably only make sense for an automatic migration, so didn't want to add it to the base method.