Migration: build system integration

[tjkirch]

• Figure out which migrations should be built and included in the image (could start with a hardcoded list we keep up to date?)
• Build the relevant migrations during the build process, each of which will be a Cargo project in a directory under workspaces/api/migration/migrations/vX.Y
• Rename the built binaries to include the version from the path (see above) to fit migration conventions. (The final name would be invalid to Cargo, so it must be a rename.)
• Install the migrations to /var/lib/thar/datastore/migrations in the image.

[sam-aws]

Are all these points covered by #125?

[tjkirch]

Yep, thanks again @sam-aws!