Implement Mixed data type

[papafe]

Add support for Mixed data type.

Fixes #2066
Fixes #2205
Fixes #2160

TODO

• Changelog entry
• Tests

[papafe]

Please note that the PR is still in draft, and that it also contains all the changes from PR #2230, as those changes are needed, and can't be merged to master yet due to a missing merge in core.

[papafe]

I have just verified that sync tests are passing. With this there is very little to do, and it depends on bugs/features in Core:

• Validate key in dictionary does not respect size (validate_key_value does not respect string length in Mixed realm-core#4589)
• Issue with realm in objectStore not containing the full schema There is a ticket (Realm instances should have a complete view of their schema even when opened as non-dynamic realm-core#4584), but it also seems there is a more complex solution for now that was implemented in Cocoa
• query.equal does not work with an object parameter (Support equal/not_equal on a link column against a Mixed value realm-core#4597)

[nirinchev]

We should also consider exposing RealmValue.ObjectType which returns AsRealmObject().ObjectSchema.Name - while users can definitely get to it themselves, I'd be quite impressed if they discover it on their own.