fix: add ConfigureAwait(false) throughout async code to prevent synchronization context deadlocks

[Banovvv]

Problem

Users calling any async method on CloudConvertAPI (CreateJobAsync(), UploadAsync(), WaitJobAsync(), etc.) in environments with a synchronization context (ASP.NET classic, WinForms, WPF) would experience a deadlock if the call chain blocked anywhere with .Result or .Wait().

Fix

• Added ConfigureAwait(false) to every await call in WebApiHandler and RestHelper, so continuations resume on any available thread pool thread rather than the captured synchronization context.
• Additionally, I refactored RestHelper.RequestAsync<T>() to short-circuit on HTTP 204 NoContent before attempting to read the response body, avoiding an unnecessary ReadAsStringAsync() call.

Fixes: #14

References

• ConfigureAwait FAQ - .NET Blog