Add CancellationToken property to FlurlCall

[trygvelo]

Hello and thank you for a great library!

When writing an event handler we discovered the need to access the CancellationToken passed into FlurlRequest.SendAsync. I can submit a PR with the change where FlurlCall has a new property CancellationToken which will be set when FlurlCall is instantiated in FlurlClient.SendAsync. Unless this is a bad idea for reasons I cannot see?

[SemaphoreSlim1]

@trygvelo I think this project is either dead or in hospice care. The maintainer has suggested this in another PR.

Independent of that, there are multiple PRs that have been open for years without any updates.
Independent of that the maintainer had almost no github activity over the last two years.
Independent of that, the last nuget package was pushed January 1 2024.

As much as I hate to say it, instead of spending your time on a PR, maybe migrate to another library. My company is.

[SemaphoreSlim1]

This request is similar to #857