Allow extensions for VSTHRD001 (Avoid legacy thread switching APIs)

[sharwell]

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

VSTHRD001 reports violations for use of specific thread switching 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 VSTHRD001.

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.