Draft: New model format
This merge updates the ketos model format.
The current strategy saves model checkpoints and a model recipe that allows ketos to rebuild the network when needed. TensorFlow has not recommended this for a while, and the current recommendation is to use the .keras v3 format.
Updating the model format will remove the main obstacle to upgrading the TensorFlow dependency to newer releases (#42), which is also holding back other dependencies (like NumPy #38).
This is also a good opportunity to add more metadata or configs to the ketos modelformat (#36)