S3 gateway fails with client cross-repository multipart copies signed with sigV2

[arielshaqed]

This combination does not work, see #7468:

• S3 gateway, performing a...
• multipart...
• copy operation...
• across buckets...
• if using sigV2...
• with the MinIO client[^*].

That PR fixed some missing fields, but something is still missing and signature validation fails. I'm pushing with this issue because it's still better than nothing, and who uses sigV2 anyway, but nonetheless this is a bug and we should fix it.

To reproduce, change TestS3CopyObjectMultipart to create its client using s3lakefsClient using credentials.NewStaticV2.

[^*]: I didn't put this only to be difficult. It's probably the only way that I know to force sigV2. But it's (probably...) not due to using MinIO client, rather to using SigV2.