Add support for custom serdes attributes

[longhuan-cisco]

sonic-swss side change: sonic-net/sonic-swss#3764

Description

Add support for custom serdes attrs SAI_PORT_SERDES_ATTR_CUSTOM_COLLECTION

In media_settings.json, custom attributes can be defined by using CUSTOM: as prefix, and existing parsing logic/format will be reused.

Example media_settings.json snippet

                    "CUSTOM:ABC": {
                        "lane0": "0x1",
                        "lane1": "0x1",
                        "lane2": "0x1",
                        "lane3": "0x1",
                        "lane4": "0x1",
                        "lane5": "0x1",
                        "lane6": "0x1",
                        "lane7": "0x1"
                    },
                    "CUSTOM:XYZ": {
                        "lane0": "0x10",
                        "lane1": "0x10",
                        "lane2": "0x10",
                        "lane3": "0x10",
                        "lane4": "0x10",
                        "lane5": "0x10",
                        "lane6": "0x10",
                        "lane7": "0x10"
                    },
                    "CUSTOM:XXX": {
                        "lane0": "0x7",
                        "lane1": "0x7",
                        "lane2": "0x7",
                        "lane3": "0x7",
                        "lane4": "0x7",
                        "lane5": "0x7",
                        "lane6": "0x7",
                        "lane7": "0x7"
                    },

Motivation and Context

How Has This Been Tested?

Tested end-to-end xcvrd->OA->SAI/SDK on 8-hostlanes optics with both non-breakout and breakout modes.

Additional Information (Optional)

[mssonicbld]

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[longhuan-cisco]

@prgeor @mihirpat1 could you please review, thanks

[mssonicbld]

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[Copilot]

Pull Request Overview

This PR adds support for custom SerDes attributes in the media settings parser, allowing custom attributes to be defined in media_settings.json with a CUSTOM: prefix and converted to JSON format for consumption by the SAI layer.

• Introduces parsing logic for custom SerDes attributes prefixed with CUSTOM:
• Adds conversion functions to handle hex/int string values and convert them to signed integers
• Implements JSON serialization of custom attributes for database storage

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
sonic-xcvrd/xcvrd/xcvrd_utilities/media_settings_parser.py	Adds custom SerDes attribute parsing logic, helper functions for string-to-int conversion, and integration into the media setting notification flow
sonic-xcvrd/tests/test_xcvrd.py	Adds unit tests for the custom SerDes attribute handling functionality

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[mssonicbld]

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[mssonicbld]

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[mssonicbld]

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