Enable Direct Access to `ConstantKeyword` from `elasticsearch_dsl `Module

[alirezaazadi]

It has been requested for the ConstantKeyword support in this issue: Support for constant_keyword #1373

The field has been added, but it's only available through this path: from elasticsearch_dsl.field import ConstantKeyword.
I wonder whether it's possible to make it accessible through from elasticsearch_dsl import ConstantKeyword like the rest of the fields? If the answer is positive, I would be happy to create a PR and apply this small change :)

[miguelgrinberg]

Yes, this makes sense, let's do it.