gbmcweb: Add Redfish BootOptions GET (collection and member)

Adds read-only Redfish support for UEFI boot options backed by
SystemRegistry:
- ComputerSystem Boot.BootOptions link (when a SystemRegistry is present).
- GET /redfish/v1/Systems/<id>/Boot/BootOptions/ returns the collection.
- GET /redfish/v1/Systems/<id>/Boot/BootOptions/<ref>/ returns a member;
  404 if the boot option does not exist.
Google-Bug-Id:539554519
PiperOrigin-RevId: 994725773
Change-Id: I18efa80e73b52e90b358824f5b1c38e2571f5b6f