Reload network configuration without agent restart

[lgfa29]

Proposal

Some CNI plugins require configuration to be dynamically applied, which means that new CNI plugins and configuration placed in the cni_path and cni_config_dir while the Nomad agent is running. Currently agents need to be restarted to reload these plugins and configuration files, which is a cumbersome process that may result in allocations being shuffled around for no good reason.

CNI network configuration should be reloadable, either through a SIGHUP or as a first-class concept that is registered in servers and propagated to clients.

Use-cases

Deploy and configure CNI plugins.

Attempted Solutions

N/A

Note
Issue extracted from #13824. Refer to that issue for more context.