This commit adds a basic itest that is run under the `test_native_sql`
flag. It tests that the migration is idempotent and that the node cant
be restarted without the `db.use-native-sql` flag once the graph
migration has been run.
This commit introduces the functionality to set a tombstone key
in the invoice bucket after the migration to the native SQL
database. The tombstone prevents the user from switching back
to the KV invoice database, ensuring data consistency and
avoiding potential issues like lingering invoices or partial
state in KV tables. The tombstone is checked on startup to
block any manual overrides that attempt to revert the migration.
