volume put: upload without sha256 hash

[danielnorberg]

Let the backend return the resulting sha256 hash instead of computing it up front.

Requires #2722 and corresponding internal change to make sha256_hex optional in the MountPutFile handler.

Describe your changes

• Provide Linear issue reference (e.g. MOD-1234) if available.

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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