Support Video Embed Field

Problem/Motivation

When video media uses the video_embed_field plugin from Video Embed Field, the URL field is not properly displayed in the "Add Media" modal.

This only affects sites with legacy media setups and is not a problem with Core media.

Steps to reproduce

- Set up a media entity that uses Video Embed Field (the YMCA Website Services distribution works too)
- Visit /admin/content/browser (this is the same widget that comes up when adding media in a block
- Add media > Video
- Observe "Video URL" shows a file picker instead of a URL field.

Proposed resolution

Add a new Form handler and use it when media entities use the "video_embed_field" plugin.

Remaining tasks

- Decide if this is worthwhile to add to the module since the situation is relatively legacy.

User interface changes

None

API changes

None

Data model changes

None.