Add generation and compression of evaluation keys at a specified level (issue #413)

[pascoec]

Implements #413: evaluation keys can now be generated with fewer RNS limbs — sized to the
ciphertexts they will actually touch — and existing keys can be compressed in place.

Draft. The core capability is complete and fully tested. Before marking ready for
review, we plan to integrate it with the bootstrapping/FBT key generation paths (see the
checklist at the bottom) so the savings apply to the largest key sets OpenFHE produces.

What this adds

API (all new arguments default to 0 = current behavior; BGV and CKKS, BV and HYBRID
key switching; BFV throws for levels > 0 per the issue):

cc->EvalRotateKeyGen(sk, {1, -1, 8}, levels);        // also EvalAtIndex/EvalAutomorphismKeyGen
cc->EvalMultKeyGen(sk, levels);                      // also EvalMultKeysGen
cc->EvalSumKeyGen(sk, levels);                       // also EvalSumRows/EvalSumColsKeyGen
cc->KeySwitchGen(sk1, sk2, levels);
auto small = cc->CompressEvalKey(evalKey, levels);   // drop limbs from an existing key

Potential Additions

• BFV support
• Bootstrapping integration: split FindBootstrapRotationIndices by transform stage inside ckksrns-fhe and generate the SlotsToCoeffs-only rotation keys at their stage's level (StC runs near the bottom of the modulus chain). Estimated ~60% off the StC subset, roughly a third of the whole bootstrapping key set; the CtS keys must stay (near) full-size since they run right after the modulus raise. Per-stage levels are derivable internally from the level budget — no user-facing API change.
• StC-first bootstrapping and CKKS FBT: in these flavors the StC transform runs on the depleted input ciphertext, so its keys can approach the size floor (a single digit over a few limbs). Apply the same per-stage generation there.
• Scheme switching (CKKS↔FHEW): the switching and rotation keys generated by EvalCKKStoFHEWKeyGen/EvalSchemeSwitchingKeyGen are applied at a fixed reduced level near the bottom of the chain — natural candidates for the levels argument.
• Documentation updates (readthedocs keyswitch/bootstrapping pages).