[Task]: Add new "enterprise" channel in devhub

[diox]

Description

Part of Enterprise Add-ons project.

Add a new "Enterprise" distribution channel mechanism (see Example UI mockup in PRD doc), behind a waffle switch (should default to off for the moment). When that channel is selected:

• We should record it as a new channel (value 3, since 1 and 2 are taken by unlisted/listed).
• --enterprise should be passed down to the linter for the corresponding upload, which will make the upload only valid if it contains the corresponding manifest property, browser_specific_settings: { gecko: { admin_install_only: true } }

Devhub version list should deal with this correctly, showing the new channel in the version list if present (probably drop the logic that hides the channel info if all versions use the same channel, to be more explicit).

Consider also showing the last enterprise version in the top-left corner of devhub dashboard as we do for unlisted/listed.

Further issues will refine that behavior but for all intents and purposes a version with enterprise channel should behave like a self-hosted (unlisted) one, not being visible to end-users anywhere, require authentication + being an author or having a permission to download.

Acceptance Criteria

### Acceptance Criteria
- [ ] New "enterprise" channel option is available when selecting distribution mechanism
- [ ] Upload is considered valid if `enterprise` channel was selected and the add-on is using the corresponding manifest property
- [ ] Devhub version list shows enterprise channel correctly if a version was uploaded for that channel
- [ ] Devhub dashboard version summary shows enterprise channel latest version if a version was uploaded for that channel
- [ ] "Enterprise" versions should not be available to the general public

Checks

• [X] If the issue is ready to work on, I have removed the "needs:info" label.

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