An attempt to add cancellation token to async methods (#41)

[AlirezaMaddah]

This is an attempt to add cancellation capability to async methods by adding the CancellationToken optional argument as follows:

1. Changed csharp.py code generator to add the CancellationToken to Async methods.
2. Changed DropboxRequestHandler to support cancellation while calling HttpClient methods and while performing the retry.
3. Changed DropboxClient to support cancellation for RefreshAccessToken.
4. Adding cancellation support to JsonWriter by using the async overloads of Newtonsoft.
5. I didn't implement the cancellation for JsonReader since it's called after the request is completed.

Checklist

General Contributing

• [x] Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• [ ] Non-code related change (markdown/git settings etc)
• [x] SDK Code Change
• [ ] Example/Test Code Change

Validation

• [x] Does this code build successfully?
• [ ] Do all tests pass?
• [x] Does Stylecop pass?

[codecov[bot]]

Codecov Report

Merging #209 (0c04841) into main (587b5e4) will decrease coverage by 0.00%. The diff coverage is 17.85%.

@@            Coverage Diff             @@
##             main     #209      +/-   ##
==========================================
- Coverage   10.17%   10.17%   -0.01%     
==========================================
  Files          17       17              
  Lines        1572     1573       +1     
==========================================
  Hits          160      160              
- Misses       1412     1413       +1     

Flag	Coverage Δ
unit	10.17% <17.85%> (-0.01%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dropbox-sdk-dotnet/Dropbox.Api/DropboxClient.cs	0.00% <0.00%> (ø)
...ox-sdk-dotnet/Dropbox.Api/DropboxRequestHandler.cs	0.00% <0.00%> (ø)
dropbox-sdk-dotnet/Dropbox.Api/Stone/Encoder.cs	14.43% <20.83%> (-0.16%)	:arrow_down:
dropbox-sdk-dotnet/Dropbox.Api/Stone/JsonWriter.cs	39.06% <29.41%> (ø)

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[AlirezaMaddah]

Hi @greg-db Would you please review this PR? I tried to be comprehensive in terms of explaining the details. Please do let me know if anything is unclear. Thanks!

[greg-db]

Thanks for putting this together! I'm not the right person to do the actual review, but I'll ask the team to review this when they can.

[AlirezaMaddah]

Thanks, @greg-db! I'm not yet familiar with all Dropboxers! :) I think @connorworley or @rogebrd could help review this.

[AlirezaMaddah]

Hi @greg-db @connorworley Is there any update on this PR?

[greg-db]

@AlirezaMaddah This is still open with the team, but I don't have an update on this right now.

[CLAassistant]

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0 out of 2 committers have signed the CLA.

:x: Alireza Maddah
:x: AlirezaMaddah

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