Add more configs to KThreesServerConfig and KThreesAgentConfig

[nasusoba]

Compared with the available config on k3s cli page, we are missing several configs (like pause-image, resolv-conf). We could do best effort for exposing these configs in KThreesServerConfig and KThreesAgentConfig, but also allow the user to add key-value like configuration to a map structure, so that if certain config is missing in KThreesServerConfig, one could still add it🤔

[nasusoba]

I find that multiple config files is already supported by k3s, one could add arbitrary configs by create a file to /etc/rancher/k3s/config.yaml.d/*.yaml. This issue is with low priority.

We could still add the configuration to KThreesServerConfig if it is commonly used.