can_capnp_to_list: direct capnp to vector<CanData> conversion

[deanlee]

Reopening #33952, which was auto-closed by the bot due to inactivity.

This PR depends on commaai/opendbc#1452.

It simplifies the data conversion process between C++ and python by directly converting Capnp data to vector[CanData] within the C++ space. Previously, the data passed between C++ and Cython involved multiple conversions, including unnecessary back-and-forth transformations between vector[CanData] and Python nested lists. This PR eliminates those redundant steps, improving performance and reducing complexity in the interaction between C++ and python.

Below is the benchmark comparison on the 3X device:

Benchmark (PR):

6000 CAN packets, 10 runs
353.46 mean ms, 359.61 max ms, 351.58 min ms, 2.14 std ms
0.0582 mean ms / CAN packet

Benchmark (Master):

6000 CAN packets, 10 runs
491.51 mean ms, 492.63 max ms, 490.43 min ms, 0.71 std ms
0.0819 mean ms / CAN packet

With the PR from commaai/opendbc#1439, the benchmark time could drop from 0.0819 ms to around 0.02 ms per CAN packet. This brings a significant performance boost, almost matching the speed of pure C++ for parsing CAN strings by minimizing the overhead between C++ and Cython.