Analyzer proposal: Async methods should take a CancellationToken

[AArnott]

Is your feature request related to a problem?

Based on the idea in DotNetAnalyzers/AsyncUsageAnalyzers#40 and discussed in #382 (comment), let's use this issue to track the proposal for an analyzer that creates a diagnostic for async methods that do not accept a CancellationToken.

Describe the solution you'd like

This could be a Hidden diagnostic with a code fix that makes adding a CancellationToken parameter easy.
It might also be a more visible Info or Warning diagnostic that activates when no CancellationToken is taken, but only after the async method calls into other code that can take a CancellationToken.

Another idea is to have an analyzer that highlights any calls to async methods where a CancellationToken that is available is not provided, but could be.

[AArnott]

Some good ideas from @sharwell when he implemented this in AsyncUsageAnalyzers: DotNetAnalyzers/AsyncUsageAnalyzers#32 (comment)

I'm liking the idea of splitting this into two analyzers:

1. Take a CancellationToken as a parameter in any public, async method, or any other method that internally calls methods that can accept a CancellationToken.
2. Propagate CancellationToken: when a method accepts a CancellationToken emit a diagnostic when it isn't propagated in invocations that may accept one (by omitting an optional parameter, or by calling a different overload). But do not emit a diagnostic for invocations that occur after any code comment that mentions "CancellationToken", supposing it explains that the method has past the point of no return. Should we produce a diagnostic even if CancellationToken.None is explicitly provided as the argument? (maybe not, but then it could be a carry-over from before the calling method accepted a CancellationToken. This would have Info or Warning severity.

[AArnott]

Revising no. 2 above consistent with an offline conversation that IIRC @sharwell and I had a while back:

Passing CancellationToken.None still gets flagged. If a method passes "the point of no cancellation" it should assign a default token to the cancellationToken local variable and continue to pass that to all async methods.