Platform.MSBuild should allow generating a helper for registration of extensions

[Evangelink]

Summary

Generate some test application builder extension that registers all extensions exposing a "hook".

Background and Motivation

Microsoft.Testing.Platform.MSBuild can auto-generate an entry point (Main) in which it will add the registration of all the extensions that define a "hook". We should offer more flexibility by allowing to generate either the entry point or an extension method.

Having this separate extension method will help with:

• xUnit v3 (cc @bradwilson)
• explicit main
• WinUI and UWP

Proposed Feature

In addition to the flag for generating the platform entry point, let's introduce another flag that allows to generate an extension method for registering the extensions exposing a "hook". I suggest to call this method AddAutoRegisteredExtensions.

Alternative Designs

• None

AB#2201333