Define custom metadata for media types without using attributes

Description

Add the possibility to define custom metadata for media types without using attributes (ImageDescriptorAttribute, MediaDescriptorAttribute) to better enable headless projects.

Metadata defined with attributes should be handled as before, but metadata defined through the API should be stored in the database. When resolving these values the information in the database should be used primarily, if no database values exist for these fields they will be resolved using the attributes as before. The metadata includes:

• Properties from the ImageDescriptorAttribute
• Properties from the MediaDescriptorAttribute