Support content type filter in x-data schema

As an application developer, For a specific x-data, I want to be able to specify a filter for content types that shall be able to use it. I want to be able to either explicitly specify content types to allow or use a regex to allow content types based on a specific pattern (see https://github.com/enonic/xp/issues/6154).

Use the mixin only on the landing-page content type :

<?xml version="1.0" encoding="UTF-8"?>
<x-data>
  <display-name>Menu</display-name>
  <allowContentType>landing-page</allowContentType>
  <items>
...
  </items>
</x-data>

Use the x-data on the landing-page content type and any content type from a specific app containing media substring:

<?xml version="1.0" encoding="UTF-8"?>
<x-data>
  <display-name>Menu</display-name>
  <allowContentType>landing-page</allowContentType>
  <allowContentType>^app.enonic.com.features:media*</allowContentType>
  <items>
...
  </items>
</x-data>

Use the mixin on all content types but only from the current app:

<?xml version="1.0" encoding="UTF-8"?>
<x-data>
  <display-name>Menu</display-name>
  <allowContentType>^{$app}:*</allowContentType>
  <items>
...
  </items>
</x-data>

Allow all content types except for the ones containing htmlarea substring:

<?xml version="1.0" encoding="UTF-8"?>
<x-data>
  <display-name>Menu</display-name>
  <allowContentType>((?!htmlarea).)*$</allowContentType>
  <items>
...
  </items>
</x-data>

Conditions of satisfaction:

I'm able to specify one or more patterns filtering allowed content types via allowContentType field in the new <x-data> schema
This does not affect the old schema setup referencing <mixin>(from site or content type directly). However, if an x-data is referenced, the filter must be applied
Implement support of {$app} macro inside the pattern to be able to refer to content types from the same app
If I add an x-data to site.xml or a content type scheme and the content type doesn't match this mixin's allowContentType pattern, then the mixin should not be rendered in the Wizard.
Filtering of content types should be covered by sufficient tests

enonic / xp

Support content type filter in x-data schema #6215