Add functionality for creating automorphism keys at a specified level

[andreea-alexandru]

Also, for a specific rotation index, keep only the key with the most towers (in the cryptcontext map with all automorphism keys).

A related issue was originally opened in palisade: https://gitlab.com/palisade/palisade-development/-/issues/90 (there was even an early implementation, but it was hacky/did not fully work). It would take an existing evaluation key and compress it to a certain level.

The desired API change is to add an extra argument to Eval*KeyGen methods, which will set the level at which the key should be created (by default, it will be 0).

It would also be helpful to add a CompressEvalKey function that reduces the number of RNS limbs for an existing evaluation key.

The initial focus should be on BGV and CKKS. The BFV setup is a little bit trickier (there, the level would imply how many RNS limbs were removed).

Unit tests should be added.

[m271828]

@shai-aws has me working on this ticket and I had an API question for you. Currently CryptoContextImpl has setters and getters for GetKeyGenLevel but the ticket mentions adding it to the actual method call. Do you want the setters and getters (that are marked for future use, not currently used) removed and have it put in the API call as mentioned here?

[yspolyakov]

Hi @m271828 The goal of the new functionality is to replace the current (undocumented) cryptocontext-level GetKeyGenLevel/SetKeyGenLevel functionality (which works only for BV key switching) with the new key-level API. I would not recommend removing the old functionality yet as it is used by some example projects, e.g., https://github.com/openfheorg/openfhe-genomic-examples/blob/main/demo-logistic.cpp. It is better to mark it as deprecated so we could remove it in a later version of OpenFHE.