[build] add extension point for consuming repositories (#412)

* [build] add extension point for consuming repositories

Context: dotnet/android#9237
Context: dotnet/android-tools@34e98e2

Using the latest .NET 9 RC 1 builds, dotnet/android repo fails to
restore with:

    external/debugger-libs/Mono.Debugging/Mono.Debugging.csproj :
    error NU1102: Unable to find package Microsoft.NETCore.App.Ref with version (= 6.0.34

To solve this, we generally set `$(RestoreAdditionalProjectSources)`
to a feed that would include the 6.0.34 runtime packages.

Add an "extension point" to mono/debugger-libs, that would allow us to
have a submodule like:

    external/debugger-libs

Then would allow us to create the file:

    external/debugger-libs.override.props

This file would set `$(RestoreAdditionalProjectSources)` until the
6.0.34 runtime is released.

* `.props`! Whoops!

Directory.Build.props

@@ -0,0 +1,7 @@
+<Project>
+  <!-- Enables git repos using debugger-libs as a submodule to override MSBuild properties -->
+  <Import
+      Project="$([System.IO.Path]::GetDirectoryName($(MSBuildThisFileDirectory))).override.props"
+      Condition=" Exists('$([System.IO.Path]::GetDirectoryName($(MSBuildThisFileDirectory))).override.props') "
+  />
+</Project>