Propagate batch_padding_policy through BatchOptions in JSV

[copybara-service[bot]]

Propagate batch_padding_policy through BatchOptions in JSV

• Propagate batch_padding_policy from BatchOptions -> BatchRunnerOpOptions -> QueueOptions.
• Temporarily convert batch_padding_policy to string for propagation. Add TODOs to use BatchPaddingPolicy enum when BATCH_DOWN and MINIMIZE_TPU_COST_PER_REQUEST are supported.

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