Make AddBuffer non async

[GarrettBeatty]

for some reason after making bc601e1, i was still able to build successfully and run unit tests successfully. but when i redownloaded the code on an ec2 instance i got. not sure why i couldnt reproduce it on my local laptop.

This change updates the code so that the function is non async since it doesnt use await at all

C:\Users\Administrator\Desktop\aws-sdk-net\sdk\src\Services\S3\Custom\Transfer\Internal\PartBufferManager.cs(289,27): error CS1998: This async method lacks 'await' operators a 
nd will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread. [C:\Users 
\Administrator\Desktop\aws-sdk-net\sdk\src\Services\S3\AWSSDK.S3.NetStandard.csproj::TargetFramework=netcoreapp3.1]
C:\Users\Administrator\Desktop\aws-sdk-net\sdk\src\Services\S3\Custom\Transfer\Internal\PartBufferManager.cs(289,27): error CS1998: This async method lacks 'await' operators a 
nd will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread. [C:\Users 
\Administrator\Desktop\aws-sdk-net\sdk\src\Services\S3\AWSSDK.S3.NetStandard.csproj::TargetFramework=net8.0]
C:\Users\Administrator\Desktop\aws-sdk-net\sdk\src\Services\S3\Custom\Transfer\Internal\PartBufferManager.cs(289,27): error CS1998: This async method lacks 'await' operators a 
nd will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread. [C:\Users 
\Administrator\Desktop\aws-sdk-net\sdk\src\Services\S3\AWSSDK.S3.NetStandard.csproj::TargetFramework=netstandard2.0]

i re-ran the build and unit tests and integration tests on the ec2 instance that was seeing this error and they all pass.

theres no dev config here since its already an existing one on this feature branch