Add pre-signed download URL for attachments to webhook payload

[liangyuanruo]

This is part 1 of the story for webhook attachments.

  const webhookData: WebhookData = {
    formId: String(this.form),
    submissionId: String(this._id),
    encryptedContent: this.encryptedContent,
    verifiedContent: this.verifiedContent,
    version: this.version,
    created: this.created,
    attachmentDownloadUrls: metadata,  // Map(fieldId -> S3 Presigned URL)
  }

We want to add a new field attachmentDownloadUrls into the above block. This object will be a JS object mapping the form’s fieldId to an S3 pre-signed URL to allow the FormSG Webhook SDK to download the object.

The S3 pre-signed URL will be generated by a new function on each call with the AWS S3 SDK, and each URL will be valid for 1 hour from the time it is generated and sent downstream.

A separate issue in opengovsg/formsg-javascript-sdk#60 has been created for the delivery of the downloadAttachments() function in the SDK.