Clean up async timeout extension methods V2

[ladeak]

• Consolidating all Timeout extension methods in Shared folder

• Adding a new compiler condition to set namespace and visibility of the TaskExtensions to distinguish http2cat and Microsoft.AspNetCore.Testing usage. Both projects reference the Shared file.

• http2cat uses namespace System.Threading.Tasks.Extensions so that the catch block in TimeoutAfter method can filter catching TimeoutException based on the Source property.

• Functional/Integration tests add file reference to Shared TaskExtensions (as they do for http2cat as well) to make http2cat\Http2Utilities.cs compile without referencing Microsoft.AspNetCore.Testing

• Removing WithTimeout, OrTimeout and DefaultTimeout definitions in other projects

• Using DefaultTimeout consistently instead of WithTimeout and OrTimeout

• Keeping Timeout duration definitions unique to projects (IIS) which has larger than the usual 30 seconds elsewhere

• Adding NETCore TargetFramwork to Microsoft.AspNetCore.Testing so .NET6 bits can be used

• Using WaitAsync in NET6 and with if-else compiler switches to fallback to existing implementation

Addresses #16620

[ladeak]

V1 version: #31671 see for further details and discussions.

[ladeak]

I will close and cancel this PR, and evolve the one referred as v1.

[ghost]

Hi @ladeak. It looks like you just commented on a closed PR. The team will most probably miss it. If you'd like to bring something important up to their attention, consider filing a new issue and add enough details to build context.