fix: etcd extra args not passed to etcd

[salasberryfin]

What this PR does / why we need it:

Extra arguments passed to etcd are defined in rke2AgentConfig which is embedded in rke2ServerConfig. The issue occurs when we generate control plane configuration and populate these two structures, here:

func GenerateInitControlPlaneConfig(opts ServerConfigOpts) (*rke2ServerConfig, []bootstrapv1.File, error) {
	if opts.Token == "" {
		return nil, nil, fmt.Errorf("token is required")
	}

	rke2ServerConfig, serverFiles, err := newRKE2ServerConfig(opts)
	if err != nil {
		return nil, nil, fmt.Errorf("failed to generate rke2 server config: %w", err)
	}

	rke2AgentConfig, agentFiles, err := newRKE2AgentConfig(AgentConfigOpts{
		AgentConfig: opts.AgentConfig,
		Client:      opts.Client,
		Ctx:         opts.Ctx,
		Token:       opts.Token,
	})
	if err != nil {
		return nil, nil, fmt.Errorf("failed to generate rke2 agent config: %w", err)
	}

	rke2ServerConfig.rke2AgentConfig = *rke2AgentConfig

	return rke2ServerConfig, append(serverFiles, agentFiles...), nil
}

rke2ServerConfig is initialized first and etcd extra values are successfully passed but then rke2AgentConfig is generated and embedded into rke2ServerConfig which empties the original value of EtcdArgs.

By moving these etcd configuration fields from rke2AgentConfig to rke2ServerConfig we can avoid these being overridden without having to apply changes to the existing logic.

Which issue(s) this PR fixes:
Fixes #234

Special notes for your reviewer:

While reproducing the bug, creating a RKE2ControlPlane with extra arguments for etcd resulted in an incorrect configuration, as validated from one of the control plane nodes:

# cat /etc/rancher/rke2/config.yaml                          
cluster-cidr: 10.45.0.0/16                                                                                                                
cni:                                                                                                                                      
- calico                                                                                                                                  
disable-cloud-controller: true                                                                                                            
service-cidr: 10.46.0.0/16                                                                                                                
tls-san:                                                                                                                                  
- 192.168.48.4                                                                                                                            
token: 7f8ddf486d5dff143dbf8964eb4f48ff 

This aligns with the findings in GenerateInitControlPlaneConfig, where the extra arguments are correctly passed to rke2ServerConfig but then deleted after initializing rke2AgentConfig.

After moving the fields to rke2ServerConfig, the final RKE2 configuration matches the expected result:

# cat /etc/rancher/rke2/config.yaml
cluster-cidr: 10.45.0.0/16
cni:
- calico
disable-cloud-controller: true
etcd-arg:
- quota-backend-bytes=5368709120
- auto-compaction-mode=periodic
- auto-compaction-retention=5h
service-cidr: 10.46.0.0/16
tls-san:
- 192.168.48.4
token: 81a3fd0a57013ff15b71a37bd3274cff

Checklist:

• squashed commits into logical changes
• includes documentation
• adds unit tests
• adds or updates e2e tests