Analyzer proposal: Async methods should take a CancellationToken

Is your feature request related to a problem?

Based on the idea in https://github.com/DotNetAnalyzers/AsyncUsageAnalyzers/pull/40 and discussed in https://github.com/Microsoft/vs-threading/issues/382#issuecomment-423817451, 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.

Start by reviewing the AsyncUsageAnalyzers pull request and the linked vs-threading discussion. Clarify whether the goal is a hidden diagnostic with a code fix, a conditional warning, or a call-site diagnostic; the work is complete only when one behavior and its scope are agreed.