Custom Content Types: list portfolio project types with thumbnails

#### Is your feature request related to a problem? Please describe.

It's possible to display specific portfolio projects by type, but it's not possible to list the project types themselves.

#### Describe the solution you'd like

Add the ability to assign a thumbnail/featured image to portfolio project types themselves, and then have a way to list these project type thumbnail images in a grid or other gallery configuration, to serve as an index for portfolio projects on a site.

#### Describe alternatives you've considered

Currently the only way to do this is by creating an image gallery with the "thumbnails", and then manually linking them to the various project type archive pages.

#### Additional context

Requested in https://wordpress.org/support/topic/any-plugin-to-display-portfolio-types-or-tags/

No files, tests, or entry points are mentioned. Start by locating the portfolio project type implementation and its archive display behavior, then determine how type thumbnails and a gallery or grid index should be represented; done means project types can expose thumbnails and be listed for navigation to their archives.

Written by the indexing model from the issue text.