Async methods should support a cancellation token

[kenkendk]

Describe the feature request or question
The ...Async methods, such as ReadSecretAsync() does not support a CancellationToken so it is not possible for the caller to cancel the request.

Link to the Vault API Docs that support this feature
Not relevant, this is purely a C# convention.

Additional context
See sonar rule here: https://community.sonarsource.com/t/c-asynchronous-methods-should-take-a-cancellationtoken/42190