[CPO] Extend ChassisBase to support CPO ports

[bgallagher-nexthop]

Description

The "Port Mapping for CPO" HLD proposes changes to the Chassis object for CPO hardware platforms, where a function called construct_cpo_devices is called when an cpo.json file is present for the SONiC device. The intent is for vendors to implement construct_cpo_devices and create the CPO platform API objects required for each interface on the vendor's switch.

Example usage, from the HLD:

class VendorCpoChassis(ChassisBase):
  ...
  def construct_cpo_devices(self, cpo_data):
      for interface_name, interface in cpo_data["interfaces"].items():
          associated_devices = interface["associated_devices"]
          assert len(associated_devices) == 2, \
              "We expect 2 devices per interface on this CPO hardware platform"

          # Construct OE/ELSFP objects
          oe = None
          elsfp = None
          for dvc_info in associated_devices:
              device = cpo_data["devices"][dvc_info["device_id"]]
              if device["device_type"] == "optical_engine":
                  oe = VendorOe(hardware_id=self.hw_id, bank=dvc_info["bank"])
              elif device["device_type"] == "external_laser_source":
                  elsfp = VendorElsfp(hardware_id=self.hw_id, bank=dvc_info["bank"])

          # Create a CpoBase-derived object to represent this interface
          assert oe, "No optical engine found for this interface"
          assert elsfp, "No ELSFP found for this interface"
          # VendorCpoSfp would be a subclass of CpoSfpBase here.
          self._sfp_list.append(VendorCpo(hardware_id=self.hw_id, oe=oe, els=elsfp))

This PR adds a construct_cpo_devices stub function for vendors to implement and calls this function when cpo.json is present. It also adds function to store and retrieve CpoBase objects on a Chassis object. Additionally, it changes CPO Platform objects to inherit from DeviceBase so that they adhere to the DeviceBase interface.

Note: this PR requires sonic-net/sonic-buildimage#28077 to be merged first.

Motivation and Context

This change is required to instantiate the required platform API objects (OE, ELSFP, etc) for a Chassis that models CPO hardware.

How Has This Been Tested?

Unit-tests have been added.

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[Natgerbi]

do we want to return an error in case the entry in specific index is "None"?

[bgallagher-nexthop]

I have copied the behaviour of get_sfp here, which is to just log an error and return None if an out-of-bounds index is used.

So for the two scenarios where an incorrect access occurs, we return None in both cases:

• index is out of bounds -> None returned
• index is for a non-CPO port -> None returned

[Natgerbi]

is the construct_cpo_devices responsible also to add the "None" for the sfps?

[bgallagher-nexthop]

Correct, the intention is that the implementation of this method should add None for any non-CPO ports

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[micas-net]

LGTM!

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