[.NET7.0] Update HttpClient instrumentation to enable activity creation using ActivitySource

[vishweshbankwar]

Starting .NET7.0, HttpClient will be able to create activities using ActivitySource if there is a listener present. We need to update the instrumentation library to make use of this approach.

At present, due to legacy way of creating activity i.e. new Activity(OperationName), SDK and instrumentation library needs to perform extra operations such as running the sampler after the activity is created and setting TraceFlags, IsAllDataRecorded and Kind properties on activity. By adding HttpClient's activitySource to SDK's listener we can avoid all these extra operations.

Since this change will only be supported in apps targeting .NET7.0 and above, we will need to first detect the targetframework at runtime and take different code path accordingly.

[vishweshbankwar]

Completed via #3415