Add dynamic flag completions to CLI

[kadinsayani]

Cobra provides support for dynamic flag completions. We can add dynamic completions for flags such as --project, --config, --mode, etc.

For example, lxc init images:alpine/edge c1 -s default --project [TAB] should complete available projects.

We can also mark flags as required for completions, specify valid file extensions for file completions (e.g. yaml for profiles), and limit flag completions to valid directory names (e.g. lxc file push|pull).