bgv-to-openfhe: populate ConvertAddPlainOp pattern

[ZenithalHourlyRate]

Otherwise the following code would fail to --mlir-to-openfhe-bgv="entry-function=func ciphertext-degree=8"

func.func @func(%arg0: tensor<8xi16>) -> tensor<8xi16> {
  %c0 = arith.constant dense<10> : tensor<8xi16>
  %0 = arith.addi %arg0, %c0 : tensor<8xi16>
  return %0 : tensor<8xi16>
}

With

error: failed to legalize operation 'bgv.add_plain' that was explicitly marked illegal when bgv-to-openfhe.