Add queue size multiplier config to BulkIndexer

[Stephanie0829]

Addresses feature request in #1027.

This pull request exposes a worker queue size multiplier config QueueSizeMultiplier, which allows user to more flexibly configure their buffer size. This is especially helpful for systems with bursty traffic.

The default value is set to 1, which sets the existing behavior as the default.

Tested by locally confirming that setting new config will increase queue capacity.

[cla-checker-service]

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[prodsecmachine]

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