Allow .NET clients to cancel streams started with StreamFrom

[kentcb]

Pull request checklist

Please check if your PR fulfills the following requirements:

• [ ] Tests for the changes have been added (for bug fixes / features)
• [X] Docs have been reviewed and added / updated if needed (for bug fixes / features)
• [X] Build (fake build or .\build.cmd) on local branch was successful

Pull request type

Please check the type of change your PR introduces:

• [ ] Bugfix
• [X] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

What is the current behavior?

No ability to pass a CancellationToken to HubConnection<...>.StreamFrom in the .NET client.

What is the new behavior?

StreamFrom now accepts an optional CancellationToken, just like other streaming methods.

Does this introduce a breaking change?

• [ ] Yes
• [X] No (the parameter is optional)

Other information

Fixes #32