lwe-to-polynomial: migrate encrypt/decrypt lowering to support RNS

[ZenithalHourlyRate]

PR #1190 migrated most of the types to NewLWEType, however, for lwe-to-polynomial, there are some non-trivial changes that needs to be done.

This also relates to #882

NewLWEPlaintext now has a PlaintextSpace.Ring (for example Z_65537 / (X^N + 1)), which is different from NewCiphertext's CiphertextSpace.Ring (typically RNS)

• lowering should convert plaintext ring to rns ring
• lowering should properly handle modulus switching from Z_65537 to Z_qi (ciphertext rns modulus), which needs mod_arith dialect support, and it is non-trivial.
• lowering should handle scaling factor for CKKS scheme.

[github-actions]

This issue has 5 outstanding TODOs:

• lib/Dialect/LWE/Conversions/LWEToPolynomial/LWEToPolynomial.cpp:47: properly lower NewLWEType (often RNS) to PolynomialType.
• lib/Dialect/LWE/Conversions/LWEToPolynomial/LWEToPolynomial.cpp:471: Remove this emitError once the pass is fixed.
• tests/Dialect/BGV/Conversions/bgv_to_polynomial/BUILD:9: support RNS lowering
• tests/Dialect/LWE/Conversions/lwe_to_polynomial/BUILD:9: support RNS lowering
• tests/Dialect/Polynomial/Conversions/heir_polynomial_to_llvm/BUILD:9: support RNS lowering

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