Manage image retrieval at the player level

[defagos]

As a developer integrating Pillarbox I want to be able to associate images with the content being played in a simple way (image or URL).

Acceptance criteria

• The player metadata API supports images associated with a URL as well.

Tasks

• [x] Move image retrieval into the player itself.
• [x] Remove from Core Business.
• [x] Add new constant for image width 720.
• [x] Check behavior with a proxy.