This allows to put keys to META partition.
META contents can be viewed with `talosctl get metakeys`.
There is not real usecase for it yet, but the next PRs will introduce
two special keys which can be written:
* platform network config for `metal`
* `${code}` variable
Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
