Fix API baseline check when building site extensions (#27841)

* Include API files when building site extensions

* Test SiteEx build on PR

* Revert test code

* Disable the check all together

* revert test change

Author: John Luo

src/SiteExtensions/Sdk/HostingStartup/HostingStartup.csproj

@@ -4,6 +4,8 @@
     <TargetFramework>$(DefaultNetCoreTargetFramework)</TargetFramework>
     <OutputType>Exe</OutputType>
     <DisablePackageReferenceRestrictions>true</DisablePackageReferenceRestrictions>
+    <!-- No need to track public APIs of this tool. -->
+    <AddPublicApiAnalyzers>false</AddPublicApiAnalyzers>
   </PropertyGroup>
 
   <ItemGroup>

src/SiteExtensions/Sdk/HostingStartup/PublicAPI.Shipped.txt

@@ -42,6 +42,7 @@
 
     <ItemGroup>
       <_TemplateFiles Include="$(MSBuildThisFileDirectory)\HostingStartup\*.cs*" />
+      <_TemplateFiles Include="$(MSBuildThisFileDirectory)\HostingStartup\PublicAPI.*.txt*" />
       <!--
         Always use Major.Minor.0 so that if we have to produce a new SiteExtension during a patch build it will still work for non-patch runtimes.
         i.e. 3.0.0 dotnet will search for 3.0.0 and below and wouldn't find a 3.0.1 folder path