Add support for media folder

[frenck]

Proposal to add support for a media folder.

The target of this folder is to share media files.

It is exposed to the Home Assistant container and add-ons are allowed to map it.

Use-cases:

• Use as the default media source path in Home Assistant.
• Allow external add-ons to use the media as well. (e.g., Plex, motionEye).

Child PRs:

• Frontend: Add snapshot support for new media folder frontend#6889
• Dev docs: Add add-on media folder map option developers.home-assistant#637

Other related repos:

• CLI: No changes needed, already compatible
• Bashio: No shapshot support, no update needed

Next steps after rollout to stable:

• Update media source integration
• Update SSH & Web Terminal add-on to access the media folder
• Update Samba add-on to access the media folder
• Update a couple of community add-ons that could benefit from access to the media folder