Show show_in_rest for exposing taxonomies, not public

[joehoyle]

Came out of #1216

[joehoyle]

@WP-API/amigos #reviewmerge

[picklepete]

Hi @danielbachhuber - I just had a read through #1895 and I'm curious how this change, adding show_in_rest, will affect existing plugins which register custom taxonomies. Is your/WP-API's position going to be that these plugins should release an update to include show_in_rest?

I only ask because we @camayak are actively building a WP-API integration (we currently rely entirely on XML-RPC for publishing) and noticed a small-ish roadblock with your old plugin, Co-Authors Plus. At the moment with XML-RPC we can simply fetch the "author" taxonomy and its terms without any trouble, but using the latest WP-API release this line of code is rejecting the taxonomy altogether.

This is obviously a quick fix, which could maybe get included in this PR, but it's just a shame that WordPress site owners will need to upgrade all plugins which register_taxonomy before using the REST API's taxonomies endpoint.

[danielbachhuber]

Is your/WP-API's position going to be that these plugins should release an update to include show_in_rest?

Yes, because they need to ensure their data is appropriately modeled before it's exposed in the REST API.

noticed a small-ish roadblock with your old plugin, Co-Authors Plus.

Co-Authors Plus is introducing its own endpoints, to ensure its data is appropriately modeled Automattic/co-authors-plus#334