v2 pages API should fetch template ID

Issue Summary

With the Confluence cloud v1 REST API it is possible to find out if a page was created from a page template and which template that was. This can be achieved by a request like this:
/rest/api/content/<page-id>?expand=metadata.sourceTemplateEntityId

The response will then contain the following information

 { "metadata": \{ "sourceTemplateEntityId": "<page-template-id>" }} 

The <page-template-id> will be the ID of the page template which was used for creating the page. If no template was used the value will be an empty string.

 However, the v2 REST API endpoints for pages do not return this information 

Steps to Reproduce

• Create page from a template
• Call /api/v2/pages/<page-id> with page id of the created page

Expected Results

Metadata sourceTemplateEntityId is present

Actual Results

Metadata sourceTemplateEntityId is not present

Workaround

Using v1 endpoint with metadata expand for template Id, i.e. 

/rest/api/content/<page-id>?expand=metadata.sourceTemplateEntityId. However this endpoint will be deprecated soon.