A new MigrationsAssembly overload available

A minor improvement (avoiding literal strings😄) with a new "underrated" overload:
```cs
 sqlOptions.MigrationsAssembly(typeof(MigrationService).Assembly) 
```

https://github.com/dotnet/aspire-samples/blob/43d1746a2e05e3b25f4dee1709b1125969e857d9/samples/database-migrations/DatabaseMigrations.MigrationService/Program.cs#L12

