Introduce Mellanox platform support for FBOSS agent and qsfp_service

[stepanblyschak]

• Change in CMake build system to support multiple target platforms
• Added MlnxSwitch and MlnxPlatform implementation into fboss/agent/hw/mlnx
• Provided configuration files to run mlnx_agent under fboss/agent/platforms/mlnx/example_configs (One is XML with port mapping configuration, second one - FBOSS JSON configuration file)
• Make qsfp_service generic to be able to support multiple Qsfp Managers
• Added Mellanox support for qsfp_service under fboss/qsfp_service/platforms/mlnx