configfile: adds meta options

[pedrolamas]

This is a follow up on https://klipper.discourse.group/t/metadata-in-config-sections/8344

My intention here is to allow users to add meta_ options to ANY section on the config with whatever value they would like.

These options will be completely ignored by Klipper and will only be available on the configfile status object, under config.<section>.meta_<name> property.

Config example:

[mcu]
serial: /tmp/pseudoserial
restart_method: Arduino
meta_test: 1

[virtual_pins]
meta_other: whatever

Moonraker output example: