Include aws-sdk-go/service/s3/s3manager for multipart uploads

[pierrebeaucamp]

As requested by your contribution checklist, this is a PR for the
addition of the s3manager library to the vendored dependencies.

This package is needed to allow for multipart uploads to S3, as
described in #788.

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Relates #788

Release note for CHANGELOG:

NONE

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

...

[ewbankkit]

Related:

• Include aws-sdk-go/service/s3/s3manager for multipart uploads #14231

[bflad]

Superseded by #14221 👍

[ghost]

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