Add self-defined template configure file for embedded flannel

[Encore222]

Is your feature request related to a problem? Please describe.
I wanna set flannel to vxlan backend and directrouting. And I also wanna the "network" field in flannel conf is auto-generated by k3s according to its cluster cidr. But in fact if I go to set vxlan backend and directrouting, the only way is to set --flannel-conf in k3s to a self-defined conf, and then the "network" field is also sel-defined, too. Since on a productive env, to define every agent's ip range is an unfriendly and uneasy work, it's more reasonable to make a self-defined template of flannel conf to instantiate a real conf by args of k3s agent.

I got this by reading the source code in pkg/agent/flannel/setup.go
`
func createFlannelConf(nodeConfig *config.Node) error {
if nodeConfig.FlannelConf == "" {
return nil
}
if nodeConfig.FlannelConfOverride {
logrus.Infof("Using custom flannel conf defined at %s", nodeConfig.FlannelConf)
return nil
}
confJSON := strings.Replace(flannelConf, "%CIDR%", nodeConfig.AgentConfig.ClusterCIDR.String(), -1)

var backendConf string

switch nodeConfig.FlannelBackend {
case config.FlannelBackendVXLAN:
	backendConf = vxlanBackend
case config.FlannelBackendHostGW:
	backendConf = hostGWBackend
case config.FlannelBackendIPSEC:
	backendConf = strings.Replace(ipsecBackend, "%psk%", nodeConfig.AgentConfig.IPSECPSK, -1)
	if err := setupStrongSwan(nodeConfig); err != nil {
		return err
	}
case config.FlannelBackendWireguard:
	backendConf = wireguardBackend
default:
	return fmt.Errorf("Cannot configure unknown flannel backend '%s'", nodeConfig.FlannelBackend)
}
confJSON = strings.Replace(confJSON, "%backend%", backendConf, -1)

return util.WriteFile(nodeConfig.FlannelConf, confJSON)

}
`

Describe the solution you'd like
We can add an additional parameter to k3s agent like '--flannel-conf-tpl' to set a template of flannel conf. And render a real conf for embedded flannel.

Describe alternatives you've considered

1. Add a new parameter as above described. (can extend to other field in flannel conf)
2. Let driectrouting be a k3s' parameter passed by user. (simple and direct, but limit)
3. Reuse the '--flannel-conf' to set a template file, and rander it no matter what it is, a normal file or a template file. Then generate a new conf for the embedded flannel. (maybe lead to confusion)

[Encore222]

@davidnuzik Do I need to commit my code？

[rbrtbnfgl]

Do we still need it? It feels like another flannel flag.

[caroline-suse-rancher]

Closing due to age, and potential redundancy - can reopen if it becomes more important.