PRO-4745: Make it possible to fetch data rather than a rendered page for any page URL in Apostrophe core

[boutell]

Summary

See changelog.

What are the specific steps to test this change?

Sites render pages as normal.

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated: I'll ask Bob
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[linear]

PRO-4745 Make it possible to fetch data rather than a rendered page for any page URL in Apostrophe core

smaccari has overridden our sendPage logic to send all of req.data as JSON instead if a certain header is present, and asked that this be added to the core.

This makes sense, but others may wish to do slightly different things, like pruning req.data, or sending it in another format.

https://github.com/apostrophecms/apostrophe/blob/main/modules/%40apostrophecms/module/index.js#L485-L487

So the change we will make is to factor out the line above that actually renders the page as an overridable method, and document this as a way to completely swap out the rendering pipeline for pages.

For consistency, a similar override point is desirable for the render-widget and render-area routes.

See original request: https://gitlab-dcadcx.michelin.net/pa/apostrophe-enhancements/-/issues/1218