Expose attachment download URL on ContentAttachment

[virtualLast]

Description

Attachment content returned from:

$contentApi->children(
    $page,
    Content::CONTENT_TYPE_ATTACHMENT,
);

is successfully hydrated as a ContentAttachment.

The Confluence API response also contains a download endpoint:

```json
"_links": {
    "self": "https://.../rest/api/content/att884637698",
    "download": "/rest/api/content/883982338/child/attachment/att884637698/download"
}

Currently the client exposes the self URL via AbstractContent::getUrl(), but the download URL is discarded during hydration.

This means consumers can retrieve attachment metadata, but cannot download the attachment itself without re-querying the Confluence REST API.

Proposed solution

Extend ContentAttachment with a dedicated download URL property.

For example:

public function getDownloadUrl(): ?string;
public function setDownloadUrl(?string $downloadUrl): self;

Hydrate the value from:
$data['_links']['download']

Notes

• This intentionally does not change the behaviour of AbstractContent::getUrl(), which correctly represents the resource's self endpoint.
• The proposal simply preserves additional attachment-specific information already returned by the API.