Add new method, `withUnsafeReaderSchema` in `AvroSchema` to allow avoiding verification exception

[cowtowncoder]

Althought verification done by AvroSchema for withReaderSchema is generally useful in finding likely problems, there may be cases where potential problem is NOT actual problem, especially for certain usage. Since this is something only caller knows, it should be separate method, to be used if caller really (thinks she) knows translation to be safe despite appearing not to be.

/cc @Hellblazer (might help to at least verify the case we talked about)