feature: Add `directdownload` request parameter to `createTemplate` API (KVM)

### The required feature described as a wish

### Summary
The `createTemplate` API does not accept `directdownload` as a request parameter. This makes it impossible for tools that create templates from a stopped VM's volume (such as HashiCorp Packer) to produce a template with the `directdownload` flag set.

### Current Behaviour
`directdownload` is only accepted as a request parameter by `registerTemplate`. `createTemplate` and `updateTemplate` both omit it from their accepted parameters, though both include it in their response payloads (reflecting the value stored on the `vm_template` record).
There is no supported API path to set `directdownload = true` on a template created via `createTemplate`.

### Expected Behavior
`createTemplate` should accept an optional boolean `directdownload` parameter (KVM only), consistent with how `registerTemplate` handles it, and persist it on the resulting VMTemplateVO record.

### Proposed Change
In CreateTemplateCmd.java, add:
```java
@Parameter(name = ApiConstants.DIRECT_DOWNLOAD, type = CommandType.BOOLEAN,
description = "KVM Only: true if template should be directly downloaded "
+ "to Primary Storage, bypassing Secondary Storage. Default is false.")
private Boolean directDownload;
public Boolean isDirectDownload() {
return directDownload;
}
```
Propagate through `TemplateProfile` and `TemplateManagerImpl.createPrivateTemplate()` - following the identical pattern already used in `RegisterTemplateCmd` and its handling.

References
- `RegisterTemplateCmd.java` - existing implementation to follow
- `CreateTemplateCmd.java` - file to modify
- Downstream: apache/cloudstack-go SDK will need SetDirectdownload() added to CreateTemplateParams once this is merged
- Downstream: hashicorp/packer-plugin-cloudstack#95 - the packer plugin feature request blocked on this change

从现有实现 RegisterTemplateCmd.java 开始，然后检查 CreateTemplateCmd.java，并通过 TemplateProfile 和 TemplateManagerImpl.createPrivateTemplate() 跟踪该值。当 createTemplate 接受可选的仅限 KVM 的布尔值、传递该值并将其持久化到生成的 VMTemplateVO 记录中时，工作就完成了。

由索引模型根据 Issue 内容生成。