[marif-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

PR commit stack:

1. [Nexthop][m4340nhp] Add the platform_manager config #1871
2. [Nexthop][m4340nhp] Add the sensor_service and led_manager configs #1872
3. This (top of stack)

• Cache the last ASIC temperature readings between SAI reads. The agent returned an empty map inside its read interval, so the ASIC temperatures in FSDB were empty most of the time.
• Start fan_service with the FSDB subscription flags, so that it reads sensor data, including the agent's ASIC temperatures, from FSDB.
• Accept the agent's asic_temp_<n>_<n> ASIC temperature sensors in the fan_service cross-config check. The agent publishes these readings, so they are not in sensor_service.json.
• Skip shutdown sensors without readings in the fan_service overtemp check. Before, one such sensor disabled the check for all of them.
• Add the m4340nhp fan_service.json and its ConfigLibTest check.

Test Plan

fan_service_sw_test and cross_config_validator_test pass, and the build-time config generator accepts the config.