This commit introduces a significant rewrite of PikPak's upload, specifically
targeting direct handling of file uploads rather than relying on the generic
S3 manager. The primary motivation is to address critical upload failures
reported in #8629.
* Added new `multipart.go` file for multipart uploads using AWS S3 SDK.
* Removed dependency on AWS S3 manager; replaced with custom handling.
* Updated PikPak test package with new multipart upload tests,
including configurable chunk size and upload cutoff.
* Added new configuration option `upload_cutoff` to control chunked uploads.
* Defined constraints for `chunk_size` and `upload_cutoff` (min/max values,
validation).
* Adjusted default `upload_concurrency` from 5 to 4.
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