Refactor MultipartUploadRequestFactory into a generic Connector factory

This CL changes `MultipartUploadRequestFactory` into
`ConnectorUploadRequestFactory` and updates relevant code to use it
adequately. This is done in preparation of further implementation of
`ResumableUploadRequest` which will require classes like
`CloudBinaryUploadService` to treat both types of requests abstractly as
`ConnectorUploadRequest`s.

Change-Id: I338e71043c228454da3897e8b92599683c54c283
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5370770


Auto-Submit: Dominique Fauteux-Chapleau <domfc@chromium.org>
Reviewed-by: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Daniel Rubery <drubery@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1272386}