Extend Payload to include references to externally stored objects (#671)

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<!-- Describe what has changed in this PR -->
Extend Payload to include references to externally stored data.
Currently the only information we would store is the size of the
externally stored object, but it will be extended in the future once
more features are added.

<!-- Tell your future self why have you made these changes -->
Payloads are limited in size, and as such require external references to
pass large amounts of data. This PR extends Payload protobuf to include
information (currently only size) about externally stored objects
associated with the Payload. One Payload will be able to reference
multiple externally stored objects in the future.

<!-- Are there any breaking changes on binary or code level? -->
No breaking changes

<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->
This change is additive, and so won't break the server

Author: simvlad