Allow extensions for VSTHRD002 (Avoid problematic synchronous waits)

[sharwell]

Is your feature request related to a problem? Please describe.

VSTHRD002 reports violations for use of specific synchronous waiting APIs. For applications that abstracted this and are working to incrementally switch to newer APIs, it would be nice to add the custom abstraction to the set of APIs recognized by VSTHRD002.

Describe the solution you'd like

A similar approach to that taken by other extensible diagnostics.

Describe alternatives you've considered

Marking the methods [Obsolete], which works to a degree but suffers from inability to distinguish #pragma warning suppress at the call sites according to the specific type of violation involved.