SONiC Platform API - Add mgmt interface status override

[lotus-nexthop]

Description

Add a way for vendors to provide an override to a management port's link status.

Motivation and Context

Nexthop's platforms have an unmanaged switch sitting between the front panel rj45 and the CPU's eth0.
As a result eth0's link state as advertised by the kernel (/sys/class/net/eth0/operstate), does not reflect the state of the rj45.

e.g.

Front Panel RJ45 (Management Port)
         |
        A
         |
    BCM53134 Switch (management switch running in unmanaged mode)
       /    \
      B      C
     /        \
    BMC    CPU (eth0)

Link A may be down (if RJ45 for example is unplugged), but link C (/sys/class/net/eth0/operstate) still says up.

There are mainly pieces in SONiC that get negatively affected by this:

1. ZTP thinks eth0 is up and wastes cycles attempting to communicate via eth0.
2. SNMP will not report the correct management port link status

get_management_port_link_status_override added by this PR allows vendors to a custom way to report the management port's state.
get_management_port_link_status_override when implemented is meant to take precedent over the kernel's advertisement of eth0's link state in /sys/class/net/eth0/operstate.

In the near term, get_management_port_link_status_override in the following two places. In the future there can be more use cases.

https://github.com/sonic-net/sonic-buildimage/blob/d0bcc88756b38a7287aaa67a785bba73194d2a01/files/image_config/monit/mgmt_oper_status.py#L41-L42

https://github.com/sonic-net/sonic-ztp/blob/c84b50a36ec02451a9fb691a7ff4de310046b9f2/src/usr/lib/ztp/ztp-engine.py#L159-L162

How Has This Been Tested?

Additional Information (Optional)

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