feat(storage-r2): client uploads using R2 multipart api

[rjgtav]

What?

Implements client-side uploads by splitting files into multiple parts and uploading each one separately.

Why?

To support uploading large files, without reaching the Worker's memory limit.

How?

By splitting the file into multiple small parts and uploading each one separately. This leverages R2's support for multipart uploads: https://developers.cloudflare.com/r2/objects/multipart-objects/

This feature was requested on Discord: https://discord.com/channels/967097582721572934/1428654256562503740/1428654256562503740