* s3api: allow unsigned SSE-C customer key headers on presigned requests
AWS requires only x-amz-server-side-encryption-customer-algorithm to be signed on presigned URLs; the key and key-MD5 headers are supplied at request time. Since #9121 rejected any x-amz-* header outside SignedHeaders, SDK-generated presigned SSE-C requests (e.g. .NET GetPreSignedUrlRequest) fail with SignatureDoesNotMatch. Exempt the customer key and copy-source key headers for presigned requests only.
* s3api: test presigned SSE-C requests carrying unsigned key headers