[RFE] Define containerd drop-in config directory

[pothos]

Current situation

Customization of containerd settings requires copying the config file and overwriting the unit

Impact

Bad UX, potentially missing updates for the copied parts

Ideal future situation

Define containerd drop-in config directory in the base config and propose it upstream, too.

Implementation options

In the upstream docs there is this example: imports = ["/etc/containerd/runtime_*.toml" ] Maybe /etc/containerd/conf.d/*.toml is better? Anyway if supported upstream this should be done by default (implicit content of the configuration, not required to be on disk?)

Additional information

[t-lo]

Note that containerd config files are shipped in our containerd sysext nowadays and are held separately from the containerd ebuild; these are here: https://github.com/flatcar/scripts/tree/main/sdk_container/src/third_party/coreos-overlay/coreos/sysext/containerd