Add script to print codebase schema

gitlab-org/build/CNG#2024 (closed)

Since it is necessary to determine the latest migration present in the codebase, but migrations are no longer sorted by timestamp, we need a way for the cloud native build script to get this information in a performant manner. Loading a MigrationContext is very slow, so I added a new script, which has no Rails dependencies, to tell us this very information.

It is fast enough to run during the build process here.

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