Allow configuring S3 request checksum calculation (blocks PBM on NetApp ONTAP S3)

Hi,

PBM cannot write to NetApp ONTAP S3. Every PutObject fails with:

InvalidArgument: x-amz-content-sha256 must be UNSIGNED-PAYLOAD,
STREAMING-AWS4-HMAC-SHA256-PAYLOAD or a valid sha256 value

aws-sdk-go-v2 defaults requestChecksumCalculation to WHEN_SUPPORTED (Jan 2025) and sends STREAMING-UNSIGNED-PAYLOAD-TRAILER, which ONTAP S3 does not support. NetApp confirms this is unsupported and still unimplemented in 9.17.1P8 (their latest), and their KB names AWS_REQUEST_CHECKSUM_CALCULATION=when_required as the workaround.

PBM offers no way to set it. Under the Percona Operator for MongoDB the backup-agent container is operator-generated with envFrom: null and only explicit PBM_* variables, so the environment variable cannot be injected, and pbm config exposes no checksum option (region, endpointUrl, forcePathStyle, bucket, prefix, credentials, maxUploadParts, storageClass, insecureSkipTLSVerify).

Tested: 2.11.0 fails, 2.13.0 fails identically, 2.8.0 works (predates the SDK default change). We are pinned to 2.8.0 in production as the only workaround.

Note the failure only affects small objects — PITR oplog chunks upload fine because they go multipart, so pbm status shows PITR healthy while snapshots fail on .pbm.init. That makes it easy to miss.

Request: a storage.s3.requestChecksumCalculation (or equivalent) setting in pbm config, so it can be set where the other S3 options live.