Fixes#1896
Enforces the S3 `5 GiB` copy source size limit across the posix and azure
backends for `CopyObject` and `UploadPartCopy`, returning `InvalidRequest` when
the source object exceeds the threshold.
The limit is now configurable via `--copy-object-threshold`
(`VGW_COPY_OBJECT_THRESHOLD`, default 5 GiB).
A new `--mp-max-parts flag` (`VGW_MP_MAX_PARTS`, default `10000`) has been added to make multipart upload parts number limit configurable.
No integration test has been added, as GitHub Actions cannot reliably
handle large objects.