Add `signUploadObjectUrls()` for generating multiple signed upload URLs

Currently, Supabase Storage provides support for generating multiple signed download URLs through: However, signed upload URLs are currently only supported for a single object through: For applications that upload multiple files, developers need to generate signed upload URLs one by one. This results in multiple API calls and makes the Storage API inconsistent with the existing download URL workflow. Would it make sense to introduce a bulk API: similar to: Expected behavior The method would: • Validate upload permissions for each requested path. • Generate upload-scoped JWT tokens (SIGNED_URL_SCOPE_UPLOAD). • Return a signed upload URL for each object. Example: Possible response: Why this could be useful Many applications support multi-file uploads (images, documents, attachments, etc.). A bulk signed upload URL API would: • Reduce API round trips. • Provide symmetry between upload and downloa…