Send Expect: 100-continue on BackbeatRoutes requests with a body

[maeldonn]

Wire the AWS SDK expect-continue middleware into BackbeatRoutesClient so the server can reject invalid PutData requests before the body is streamed.

Issue: CLDSRVCLT-14

[bert-e]

Hello maeldonn,

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[bert-e]

Waiting for approval

The following approvals are needed before I can proceed with the merge:

• the author

• 2 peers