Dump media_settings from appDB during fast-reboot

[tjchadaga]

What I did

During fast-reboot, media settings will be dumped from appDB to media_config.json file so they can be loaded to redis after fast-reboot. This is done as part of fix for sonic-net/sonic-buildimage#9165.

How I did it

During fast-reboot, delayed serdes programming done after transceiver detection results in additional interface flap and traffic loss exceeding the required 30s. This is addressed by

1. making a backup of media settings from appdb during fast-reboot (This PR)
2. loading this to redis after the switch boots up (Fix for additional interface flap during fast-reboot on platforms due to delayed serdes programming sonic-buildimage#9166).
3. Configuration of serdes triggered by reading media_settings.json file is skipped (Skip loading media config from media_settings.json for fast-reboot sonic-platform-daemons#221)

How to verify it

The script will dump an additional json file (media_config.json)

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

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