Add _links to the WP-REST-API JSON index.

[jeremyzahner]

To properly implement the HAL-JSON standard, the index of the API should provide a _links section on top level so we could fetch them in a HAL way.

This goes according to RFC6570.
​https://tools.ietf.org/html/draft-kelly-json-hal-08#section-5.1

The proposal:

• The add_index_links function should create a named routes index for all top level collections (like posts, pages, comments).
• One property of the _links object shall be href (which is either templated or not).
• If the linked endpoint has query parameters, the templated property should be true and the href should contain an RFC6570 URI-Template.

The benefit:

• While aligning to the HAL-JSON RFC spec. as good as possible, we could use all kinds of client side libraries.

We will provide a patch for this functionality shortly.

[maennchen]

Prototype Patch

[jeremyzahner]

@maennchen Thanks! ;-)

[maennchen]

There seems to be no effort to be HAL compatible (See #37182). I think we can close this.

[joehoyle]

Per @maennchen's suggestion, closing.