Add DebuggerDisplay to Activity and friends

Activity doesn't have a great debug experience:

![image](https://github.com/dotnet/runtime/assets/303201/c434f586-2d58-45d5-9320-c0638852fedb)

Add `DebuggerDisplay` and `DebuggerTypeProxy` to make it easier to discover important information. For example, `OperationName` in the debugger display. And make it easier to view baggage and tags set on an activity.