test: add LeNet MNIST CNN model e2e test (small)

[copybara-service]

test: add LeNet MNIST CNN model e2e test (small)

[j2kun]

On my system it's using OpenMP and the individual calls to the OpenFHE API are quite a bit faster than we observed internally. I cut the openfhe IR at ct_9600 (just returned that SSA value directly, changed the function signature appropriately, and removed the decryption step) and the runtime of the encrypted main func is 81 seconds.

[j2kun]

After trying to run the full model I ran into:

terminate called after throwing an instance of 'lbcrypto::OpenFHEException'
  what():  external/openfhe/src/pke/include/cryptocontext.h:l.315:TypeCheck(): Null Ciphertext