[ci] Ensure all namespaces are verified in published-namespaces.txt.

[jpobst]

Context: dotnet/android-libraries#558

One task that requires extra time and care is validating that any newly bound namespaces are correctly renamed to our established standards. If one is missed, it cannot be fixed without breaking the ecosystem around it. Thus this is a task that CI should enforce because humans make mistakes.

In order to do so, we will maintain an allow-list of published namespaces that needs to be updated with any allowed namespace. This file will be published-namespaces.txt in the repo root.

After the repo assemblies has been compiled, this file can be checked with:

dotnet cake utilities.cake -t=verify-namespace-file

If there are differences, they will be reported and the build will fail:

========================================
verify-namespace-file
========================================
New Namespaces
--------------
AndroidX.Sqlite.Db
AndroidX.Sqlite.Db.Framework

Removed Namespaces
------------------
AndroidX.Biometric2

This file will need to be updated when any namespaces are added or removed. This can be accomplished with:

dotnet cake utilities.cake -t=generate-namespace-file

CI Example: