Return `post_types` field in templates REST API

[oandregal]

While working on the site editor templates screen, I realized we don't expose the post_types field through the templates endpoint ​https://developer.wordpress.org/rest-api/reference/wp_templates/, though it's one of the parameters consumers can use to query it.

In the dataviews-powered page, we want to expose this information so the user can filter templates that are intended to work with a given post type.

My suggestion is to expose this new field through the existing templates REST API endpoint.

[oglekler]

@spacedmonkey can you, please, take a look. I have doubts that we will be able to make in into 6.6, but it looks simple enough and quite reasonable to have. Thank you!

[spacedmonkey]

@oandregal can you add a unit test for this change.

[oglekler]

We have 3 hours until the commit freeze before Beta 1 release and no Unit tests. I am rescheduling it to the next milestone. Let's make it when Trunk will be open for enhancements again, without waiting for the last moment. Thank you 😅

[peterwilsoncc]

The first beta is due in under 20 hours so I'll move this off the milestone as there hasn't been any progress during the release cycle.

Trac Ticket: Core-61308

## Overview

This pull request adds a new post_types field to the existing WordPress REST API endpoint for templates (/wp/v2/templates). This enhancement allows users to filter templates based on the post types they are intended for, thereby improving usability and functionality in the site editor templates screen.

## Changes Made

### Modified get_item_schema():

• Added a new property, post_types, to the JSON schema for templates.

• The post_types field is defined as an array of strings, describing the associated post types for each template.

### Updated prepare_item_for_response():

• Implemented logic to fetch and include the post_types information in the API response.

• Ensured that the post_types field is populated based on the associated post types stored in the template's metadata.

## Benefits

• Improved Filtering: Users can now filter templates directly by their associated post types, making it easier to find relevant templates for specific content types.

• Enhanced API Usability: By exposing the post_types field, we align the API capabilities with user expectations and common use cases.

Changes look good. @Debarghya-Banerjee Would you be able to extend the existing unit tests?

Hi @martinkrcho , I have fixed the failing test cases. Can you please check it. Thanks.