Add an DownloadSignatureRequestFilesAsync method

[Sora2455]

An async version of DownloadSignatureRequestFiles would mean that servers can take advantage of C# async/await functionality instead of "Busy waiting" on the API.

Actually, async versions of all of the API endpoints would be fantastic, but that one in particular would help a ton (as it usually takes multiple seconds to run).

[janetanne]

Hi @Sora2455! Thanks for the suggestion. Our team will look into it.