gbmcweb: Add Redfish BootOptions POST

Adds POST /redfish/v1/Systems/<id>/Boot/BootOptions/ to create a UEFI boot
option via SystemRegistry::AddBootOption.

- Required: BootOptionReference (must be a valid "Boot####", validated with
  parseBootReference) and a non-empty DisplayName; otherwise 400.
- Optional: BootOptionEnabled (defaults to true) and UefiDevicePath.
- Returns 201 Created with a Location header on success, 409 Conflict if the
  boot option already exists, and 500 for any other registry error.

Google-Bug-Id:539554519
PiperOrigin-RevId: 995262773
Change-Id: I31ca43d45049d2b22f99b6a26a981684a4a8eb4e