Expose TUS upload chunk size

[tim-kos]

What changed

• Adds tusUploadChunkSize to the Transloadit initializers and passes it through to TUSKit.
• Allows apps to pass 0 to let TUSKit upload each file in one request, which can be useful for iOS background uploads where scheduling a new task for each 500 KiB chunk can stall.
• Clarifies that createAssembly(... andUpload ..., completion:) fires after the Assembly is created and uploads are scheduled, not after file upload or processing completion.
• Documents the recommended lifecycle: use TransloaditFileDelegate.didFinishUpload for upload completion and pollAssemblyStatus for processing completion.
• Adds tests for the default chunk size, a custom chunk size, and the injected-signature initializer.

Why

A customer using TransloaditKit 3.5.0 saw didFinishUpload() fire while API2 had already aborted the Assembly after seeing only three default TUS chunks. Exposing the chunk size gives apps a way to avoid many small background URLSession tasks, and the docs now make the upload lifecycle explicit.

Validation

• swift test

[kvz]

@donnywals can you review this in hindsight please

[donnywals]

Looks okay overall with one caveat; if we decide to change the default chunk size in TUSKit, we have to also update TransloaditKit with the same value (the 500 * 1024 value is currently the default in TUSKit).

One way to avoid this is to make nil the default chunk size instead of 500 * 1024. If the user didn't pass a chunk size, we don't pass a chunk size to TUSKit and we get its default. For any non-nil value we pass the user's choice explicitly.

It'd roughly look like this when building the TUSKit client:

lazy var tusClient: TUSClient = {
  let tusClient: TUSClient
  if let tusUploadChunkSize {
    tusClient = try! TUSClient(
      server: URL(string:"https://www.transloadit.com")!,
      sessionIdentifier: "TransloadIt",
      sessionConfiguration: tusSessionConfig,
      storageDirectory: storageDir,
      chunkSize: tusUploadChunkSize
    )
  } else {
    tusClient = try! TUSClient(
      server: URL(string:"https://www.transloadit.com")!,
      sessionIdentifier: "TransloadIt",
      sessionConfiguration: tusSessionConfig,
      storageDirectory: storageDir
    )
  }
  tusClient.delegate = self
  return tusClient
}()

I think there's an argument to be made for the current approach being more explicit, and for each SDK to control its own default, I think it's important to make this decision and to decide whether that's the behavior we want, or if we want to consider the user's choice to not set a chunk size to equal "let TUSKit decide".