[hssyoo]

Should fix:

• fix(s3): support proxied CRT requests and standardize error XML supabase/storage#1606
• tsh aws s3 cp gives GetObject operation: Forbidden - potentially due to CRT transfer client change gravitational/teleport#69876
• fix(ci): use classic s3 transfers for r2 artifacts okou-ai/okou#39951

The CRT request serializer added "Content-Length: 0" to any request botocore left without a Content-Length and a body. Botocore already sets Content-Length on every non-GET/HEAD/OPTIONS request, so this only ever touched GetObject. aws-c-s3 treats a missing Content-Length as zero for GET_OBJECT and DEFAULT meta requests, and copies the initial headers onto every part GET and its HEAD, so the header only added a signed "content-length" to each of those requests.

Not immediately clear why this was originally added, but there weren't any observed regressions when testing against live S3 after removing. CRT team also confirmed CRT shouldn't need it.
History:

• Originally added this in an unmerged PR for upstream s3transfer: Add streaming fileobj support to CRTTransferManager boto/s3transfer#277
• Pulled it into AWS CLI: [v2] Add support for stdin/stdout streams for CRT client #8291
• Backported it to upstream s3transfer: Support file-like objects in CRT transfer manager boto/s3transfer#279