SD Write Card Pipeline

High-level, the goal is to be able to transfer data from Harp write message to the SD Card.

## Details

The SD card operates best on sectors of 512 bytes, so it would be advantageous to buffer writes to the SD card except for the last write. We additionally need to do this in a way where we can also compute the SHA256 hash https://github.com/AllenNeuralDynamics/harp.device.quad-dac/issues/27 for each file.

Data will arrive from an extended-format Harp WRITE request.

Here's a speculative architecture diagram:

Since we're only writing data to the card one file at a time, we can reuse the resources above needed for any channel.