2-phase attachment upload uses AttachmentUploadDescriptor which allows PUT and custom header passing (includes breaking changes)

[jiwon-oai]

Add Attachment.upload_descriptor which specifies the upload method (post vs. put) and any custom headers to pass along.

Breaking:

• Removed Attachment.upload_url
• When handling AttachmentsCreateReq for phase 1 of a 2-phase upload, ChatKitServer calls await self.store.save_attachment(attachment, context=context) after calling attachment_store's create method. Previously it was the responsibility of attachment_store to save the attachment to the data store, which forced the attachment store to have access to / depend on the thread data store.