Board data: Add support for APx without sideband
Following discussion with the APx development team, to support the exchange of board data files for slice tests, this branch adds support for reading & writing APx board data files without sideband information. Notably, this required:
- Updating the relevant read & write functions
- Adding the set of defined fields to the channel data class, along with a set of required fields for each board data file format
Alongside this I added the convert-board-data
entrypoint with a --add-packet-structure
flag to support setting the relevant fields when converting from APx to EMP/X2O format.