Trim more Http DiagnosticsHandler code (#39525)

Trim more Http DiagnosticsHandler code (#39525)

Since the DiagnosticsHandler still gets instantiated in the HttpClientHandler, none of its overriden methods are getting trimmed. This leads to System.Diagnostics.DiagnosticListener still being preserved in a linked application.

The fix is to split DiagnosticsHandler.IsEnabled() into two methods:

* IsGloballyEnabled() - checks the AppContext switch, and is replaced by the linker by a feature swtich.
* IsEnabled() - which checks IsGloballyEnabled() and if there is an Activity or listener available.

This allows all but the IsEnabled and IsGloballyEnabled methods to get trimmed on DiagnosticsHandler.

Contributes to #38765