Refactoring SRG content loading

[StaehliJ]

As a integrator I want to easily play mixed content like urls and SRG contents.

Acceptance criteria

• Adding additional custom supported content is easier.
• SRG content are created with a mime_type and an uri formatted with the url of the media composition.
• SRG content can load prod, stage content in the same player.

Tasks

• Remove custom loading media item from Pillarbox and create directly a MediaSource threw suspend function.
• Improve adding MediaSource.Factory to Pillarbox.
• Provide default factory for SRG