Confusing S3Config enable_write_with_append

https://github.com/apache/opendal/blob/cf3e44fffb9f3871b4c496ffe1a9456f8e420f98/core/src/services/s3/config.rs#L189-L190

IIUC even if `enable_write_with_append` is `true`, one should still call `write_op.append` to issue an append write.

And this option is by default set to `false`.

Maybe we can inline it always with `true` since no reason we forbid our users from using S3's append functionality?

cc @Xuanwo

Start at core/src/services/s3/config.rs lines 189-190 and trace how enable_write_with_append relates to write_op.append. Verify the current default and call requirements against the issue’s S3 behavior, then define and test the resulting configuration semantics.

Written by the indexing model from the issue text.