Hydrate secrets in parallel during Sandbox.exec

[mwaskom]

Describe your changes

Noticed this while migrating .resolve -> .hydrate.

If passing a large number of secrets, this could potentially reduce the latency of Sandbox.exec.

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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[pawalt]

Awesome thanks!