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]

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[bert-e]

Waiting for approval

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• the author

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[SylvainSenechal]

finishing some stuff on another cloudserverclient pr, will recheck and try to integrate later

[maeldonn]

Should the default for expectContinueHeader stay true (matches AWS S3 client, fail-fast at any size), or switch to a small threshold like 1024 to skip the handshake on tiny payloads? Leaning true for consistency, but happy to switch.