[202405] Add GCU standalone wheel package (backport of #4310)

[yejianquan]

Backport the GCU (Generic Config Updater) standalone wheel packaging from master PR #4310 to the 202405 branch, adapted for 202405 API compatibility (following 202412 PR #352 structure).

Changes:

• Add generic_config_updater/main.py: standalone GCU entry point with all apply-patch orchestration logic (validate, preprocess, scope splitting, parallel dispatch)
• Add generic_config_updater/setup.py: builds sonic-gcu wheel with gcu-standalone console_scripts entry point
• Add generic_config_updater/pytest.ini and .coveragerc for test/coverage
• Modify field_operation_validators.py: inline DEFAULT_SUPPORTED_FECS_LIST to remove utilities_common dependency (wheel isolation)
• Add comment in utilities_common/constants.py noting the duplicate
• Modify azure-pipelines.yml: add GCU wheel build + publish step
• Add tests/generic_config_updater/main_test.py: 88 unit tests adapted from master (removed --time and --path-trace features not in 202405)

Testing performed:

• Wheel build: sonic_gcu-1.0.0-py3-none-any.whl (48KB) builds successfully
• Wheel install: gcu-standalone binary at /usr/local/bin/gcu-standalone
• All 7 CLI sub-commands verified: apply-patch, replace, save, rollback, create-checkpoint, delete-checkpoint, list-checkpoints
• All flags tested: --verbose, --dry-run, --ignore-non-yang-tables, --format, --parallel
• Basic operations: add, replace, remove patches via gcu-standalone
• Mixed operations: checkpoint create -> apply patch -> rollback -> delete
• Error handling: missing file -> exit 1, invalid patch -> proper error
• Regression: existing 'config apply-patch' CLI unaffected
• Unit tests: 88/88 pass, flake8 clean
• Device: str3-7800-lc3-1 (multi-ASIC Arista 7808, SONiC 202405)

What I did

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Previous command output (if the output of a command-line utility has changed)

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