Allow adding FITS cards to pfsConfig PHDU

Per INSTRM-1863, we need to add a handful of cards to the PHDU of the pfsConfig files.

From Slack discussions, I believe we agreed to add a header=[Dict[name, (value, comment)]] argument to the pfsConfig constructor. When the .write() method is called, those cards will be written to the PHDU.

Side note: for now we will add a PROP-ID proposal card. Once the requirements are understood, there will likely be a new proposal column in the config table, giving one proposal per fiber.