Adding DebugProvider to help override Write and ShowDialog behavior (dotnet/coreclr...

Adding DebugProvider to help override Write and ShowDialog behavior (dotnet/coreclr#20419)

* Adding DebugProvider to help override Dialog Write and ShowDialog behavior.

- New SetProvider returns previous DebugProvider while exchanging to new one
- Will keep the s_WriteCore delegates for tests until the next PR to fix indentation is merged

Related to: dotnet/corefxdotnet/coreclr#3708, dotnet/corefxdotnet/coreclr#31003

* Move ShowDialog and Write logic from Debug to DebugProvider

* disabling DebugTests

Commit migrated from https://github.com/dotnet/coreclr/commit/961f131758e510927676e13bc2f4533b3e0cb42a