Missing method from interface

[theolivenbaum]

Minor thing I just noticed, but the GetAsync method is missing from the IAttachmentsResource interface.

 public async Task<Attachment> GetAsync(
            long attachmentId,
            CancellationToken cancellationToken = default(CancellationToken))

[mikerogers123]

Hi @theolivenbaum, our team has recently taken some level of ownership of this repository. I have added a TODO card for this. We plan on prioritising the work as part of our workflow so we should get round to it. Until then, PRs are welcome 👍