Support containerd embedded

[richardcase]

Describe the solution you'd like:

containerd can be embedded into another application. We could consider publishing a version of flintlock with containerd embedded instead of requiring it to be installed already.

This could be a different binary, perhaps flintlockd-ctr

Why do you want this feature:

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]