Add endpoint for retrieving "folder" media types.

[kjac]

Prerequisites

• I have added steps to test this contribution in the description below

Description

This adds a new endpoint for retrieving the media types that are considered "folder" media types.

A "folder" media type has the following characteristics:

1. It has no "umbracoFile" property.
2. It has allowed child content types.

For backwards compatability however, the system "Folder" media type should always be considered a "folder" media type, no matter if it does match the characteristics above.

Testing this PR

Use Swagger to test the new endpoint:

• In a default install, the endpoint should yield only the system "Folder" media type.
• If you add more media types that match the above-mentioned characteristics, the endpoint should include these.
• If you change the "Folder" media type so it does not match the expected characteristics, it should still be included in the endpoint output.

[Zeegaan]

Looks good, tests good 🚀