Pass executor template configuration through the ApplicationGroup spec

Currently the default executor is hardcoded,

```go
func defaultExecutor(timeout time.Duration) v1alpha12.Template {
executorArgs := []string{"--spec", "{{inputs.parameters.helmrelease}}", "--timeout", timeout.String()}
return v1alpha12.Template{
Name: helmReleaseExecutor,
ServiceAccountName: workflowServiceAccountName(),
Inputs: v1alpha12.Inputs{
Parameters: []v1alpha12.Parameter{
{
Name: "helmrelease",
},
},
},
Executor: &v1alpha12.ExecutorConfig{
ServiceAccountName: workflowServiceAccountName(),
},
Outputs: v1alpha12.Outputs{},
Container: &corev1.Container{
Name: "executor",
Image: "azureorkestra/executor:v0.1.0",
Args: executorArgs,
},
}
}
```

Rather than using a static template we should pass the container configuration through the `ApplicationGroup` spec.