Consider adding FxCop

[lukebakken]

As suggested by @danielmarbach in this comment:

Context capturing is applied to the continuation which is the end of the method. Though in general libs that don't need context capture should always opt out for every await. It would be best to even enable i.ex. Fxcop even if only https://docs.microsoft.com/en-us/visualstudio/code-quality/ca2007?view=vs-2019 is activated

[danielmarbach]

PR added #932

For now, I only enabled the ConfigureAwait rule and disabled all the others. Once it is in we can start enabling more rules gradually