Support Audio assets

Problem/Motivation

Currently the only supported asset types are:

• Document
• Image
• Video

It would be nice if Audio would also be supported.

Proposed resolution

• Update the configuration to allow the "bynder type" field to have an audio option.
• Add a field formatter for Audio assets
• Allow the API to return audio assets.
• Add tests?

Remaining tasks

Create a working merge request.

User interface changes

• Audio is added as selectable media type.
• Audio is an configurable option for entity browsers.
• Audio is browsable through the API.

API changes

N/A

Data model changes

Extra configuration item.