Support Single part PutFile

[nirsagi]

Add env var for single part support.
AddFile will have the option to send single part. for backend which doesn't support multipart.
in order to use it change ObjectStoreConfig.Disable.Multipart value to true

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This change is 

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[k8s-ci-robot]

Hi @nirsagi. Thanks for your PR.

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[nirsagi]

@googlebot I signed it!

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[Ark-kun]

/ok-to-test

[nirsagi]

/retest

[nirsagi]

/assign @IronPan

[IronPan]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: IronPan

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[Ark-kun]

/retest