feat: add FirebaseStorageUISwift product for Swift SPM clients

[demolaf]

Closes #1271

Swift SPM clients currently can't call sd_setImage(with:) passing a StorageReference — the ObjC headers use @class forward declarations (required to fix the Clang scan-phase failure in #1350), which leaves StorageReference opaque to the Swift importer.

This PR adds FirebaseStorageUISwift, a new Swift-only SPM library product that bridges this gap by providing properly-typed Swift extensions on UIImageView.

Also migrates FirebaseUI-demo-swift from CocoaPods to SPM and updates StorageViewController to demonstrate the new API.

New product

Add FirebaseStorageUISwift to your SPM dependencies and import FirebaseStorageUISwift. This re-exports both FirebaseStorageUI and FirebaseStorage, so no additional imports are needed.

API Changes

extension UIImageView {

  // Without progress — trailing closure resolves to completion
  public func sd_setImageWithStorageReference(
    _ storageRef: StorageReference,
    maxImageSize size: UInt64? = nil,
    placeholderImage placeholder: UIImage? = nil,
    options: SDWebImageOptions = [],
    context: [SDWebImageContextOption: Any]? = nil,
    completion: ((UIImage?, Error?, SDImageCacheType, StorageReference) -> Void)? = nil
  )

  // With progress
  public func sd_setImageWithStorageReference(
    _ storageRef: StorageReference,
    maxImageSize size: UInt64? = nil,
    placeholderImage placeholder: UIImage? = nil,
    options: SDWebImageOptions = [],
    context: [SDWebImageContextOption: Any]? = nil,
    progress: ((Int, Int, StorageReference) -> Void)?,
    completion: ((UIImage?, Error?, SDImageCacheType, StorageReference) -> Void)? = nil
  )
}