Allow customizing divideAndConquerThreshold in TransferManagerConfig

[t-peoples]

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Is your feature request related to a problem? Please describe.

divideAndConquerThreshold is set to 128 MiB in DefaultQoS. There's no way to change this from any public methods. Our use case requires downloading files of arbitrary sizes concurrently, some < 128 MiB, some < 1 GiB, and some > 10+ GiB. We see that divideAndConquerThreshold adds overhead for these medium sized files.

Describe the solution you'd like

Allow defining divideAndConquerThreshold via public methods in TransferManagerConfig.

Describe alternatives you've considered

There's no way to accomplish this currently from what I can tell.

Additional context

Is there a reason 128 MiB was chosen?