Engine builders move from pkg/cmd/datastore into their engine packages,
registering themselves via the new leaf package pkg/cmd/datastore/dsconfig,
which also takes over the Config type and its options; pkg/cmd/datastore
re-exports everything, so callers are unaffected. Binaries link all in-repo
engines through a blank import of internal/datastore/engines.

This removes the static engine imports from the server-construction path,
so that shared test code can stand up a SpiceDB server without creating an
import cycle with the engines' own tests. Adding a new engine no longer
requires editing pkg/cmd/datastore.

Migratable engines can now list every migration they support, in order, and
declare the earliest migration the current code can operate against. The
datastore test suite's TestMigration uses this to apply every migration one
at a time from an empty database, standing up a SpiceDB server at the
earliest supported revision and verifying schema writes and reads through
its API across each remaining migration.

Callers of pkg/cmd/datastore.NewDatastore had to blank-import
internal/datastore/engines to use any built-in engine, which consumers of this
module could not do at all: the package is under internal/. The engine imports
move back into pkg/cmd/datastore, so importing it is once again enough, and
internal/datastore/engines and its blank imports are gone.

That was only possible because the datastore test suite reached the server
through pkg/cmd/server, so an engine's own tests could not import the suite
while pkg/cmd/datastore linked the engines. TestMigration now builds the
datastore from the engine's registered builder in the leaf package dsconfig and
serves SpiceDB's schema service in-process, keeping pkg/datastore/test free of
pkg/cmd/datastore and pkg/cmd/server.

Engines defined outside this repository keep registering themselves through
dsconfig.RegisterEngine and still get TestMigration from the suite.