Enable resumable uploads for string content.

This change introduces support for using the resumable upload protocol for string-based content, such as pasted images, behind the `kDlpScanPastedImages` feature flag. A new constructor and factory method are added to `ResumableUploadRequest` to handle string data. The `SendContentNow` method is updated to use `AttachStringForUpload` when performing a resumable upload of string content. The `BinaryUploadService::Request::FinishRequest` now checks if the callback is valid before running it.

Bug: 428221484
Change-Id: I31a2eb71d2d4511e0fde36968f1e78d35a046626
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6895479
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Reviewed-by: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Nancy Xiao <nancylanxiao@google.com>
Commit-Queue: Michał Kaczmarczyk <mickaczmarczyk@google.com>
Cr-Commit-Position: refs/heads/main@{#1512567}