fru: server 3.0: Add JSON support for CXL

Summary:
Add the `cxl` JSON boolean property to force set a slot as a CXL
device.

Tested:
All unit tests passed. With a CXL device not installed on a platform30,
unit, the CXL bit was set for the slot.

Google-Bug-Id: 435526024
Change-Id: Ife39640f2ba3f606a58ec16f1ccba7113124be49
Signed-off-by: Kyle Nieman <kyle.nieman@fii-na.corp-partner.google.com>