Add support for update document via a data stream

[martijnvg]

Currently updating documents that are part of a data stream can’t be performed via the data stream. This can only be performed via the backing index a document resides in. From the client side this information can be gathered so that the right backing index is picked for an update. However this is far from ideal. The data stream primitive should provide this functionality.

This issue is here to track the status of this feature request and is currently just a placeholder. This issue will be updated when there is an initial design plan for this feature and then track the development of this feature request.

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)