Add API for serializing bootstrapping keys

[sarojaduality]

We have APIs for SerializeEvalMultKey and SerializeEvalAutomorphismKey. We should add one for bootstrapping keys. Maybe it is possible to serialize bootstrapping keys with the SerializeEvalAutomorphismKey API, but we need to double check.

[lorenzorovida]

Related to this, in case of CKKS, I think what is missing are APIs to quickly (de)serialize m_bootPrecomMap, in FHECKKSRNS.
Rotations needed by the bootstrapping are indeed included in the (de)serialization of the rotation keys.

[andreea-alexandru]

As an initial step, we should expose a functionality to get the automorphism indices required for bootstrapping (with specified parameters).