[ChassisBase] Add get_media_settings_key() override hook

[jemston-CLS]

Add an optional get_media_settings_key(physical_port, transceiver_dict, port_speed, lane_count) hook to ChassisBase, returning None by default. Platforms may override it to derive a custom media_settings.json lookup key. The default None preserves existing behavior on all platforms.

Description

Add an optional get_media_settings_key(self, physical_port, transceiver_dict, port_speed, lane_count)
method to ChassisBase. It returns None by default. Platforms may override it to derive a
custom key used to look up SerDes SI settings in media_settings.json. The four parameters
mirror xcvrd's module-level media-settings key builder so a platform override is not limited
in what it can key on.

Motivation and Context

xcvrd resolves SerDes SI media settings from media_settings.json using vendor / media /
medium-lane-speed keys. Some platforms need a platform-vendor-defined key that these fixed keys
cannot express. This adds a chassis-level hook so a platform can supply its own key without changing
the common resolution logic. Returning None by default keeps behavior identical on every
existing platform.

How Has This Been Tested?

Added test_get_media_settings_key asserting the base class returns None
(chassis = ChassisBase(); assert chassis.get_media_settings_key(0, {}, 100000, 4) is None).

Additional Information (Optional)

This is the base hook only; the consuming change lives in sonic-platform-daemons (xcvrd) .
No platform currently overrides the hook, so no platform behavior changes as of now.

[azure-pipelines]

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[mssonicbld]

/azp run

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[mssonicbld]

Hi, there are workflow run(s) waiting for approval, you may be first-time contributor. I will notify maintainers to help approve once PR is approved. Thanks!

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