code: public AwsS3SignPolicyResult signPolicy(
        @Nonnull final String bucket,
        @Nonnull final String key,
        @Nonnull final String acl,
        @Nonnull final String contentType
    ) {
      return proxy.signPolicy(bucket, key, acl, contentType);
    }
docs: Signs an AWS S3 security policy for a future put object request. This future request would
  be made outside of the Stitch SDK. This is typically used for large requests that are better
  sent directly to AWS.
  @see <a href="https://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-post-example.html">Uploading a File to Amazon S3 Using HTTP POST</a>
  
  @param bucket the bucket to put the future object in.
  @param key the key (or name) of the future object.
  @param acl the ACL to apply to the future object (e.g. private).
  @param contentType the content type of the object (e.g. application/json).
  @return the signed policy details.