Add support for selective serialization/deserialization of automorphism/rotation keys

[yspolyakov]

This is useful to control the memory footprint for scenarios with a large number of automorphism keys, e.g., in applications using CKKS bootstrapping. Only the automorphism keys needed for a particular computation can be loaded into the memory.

[yspolyakov]

Use a separate a file for each automorphism key