[Task]: Support new "enterprise" channel in add-on API

[diox]

Description

Part of Enterprise Add-ons project.

Follow-up for https://github.com/mozilla/addons/issues/14849

The new "enterprise" channel should be supported by the API (allowing filtering in the version list, but also upload using enterprise as a channel value - Validation rules from https://github.com/mozilla/addons/issues/14850 should apply).

Enterprise versions should behave like a self-hosted (unlisted) versions and not be available to the general public through the API. The same permission used for unlisted should be used to gate accessing those versions for non-authors.

Acceptance Criteria

  ### Acceptance Criteria
  - [ ] API supports filtering versions endpoint to show only enterprise versions
  - [ ] API supports uploading new enterprise versions
  - [ ] API doesn't show enterprise add-ons by default anywhere, requires permission similar to unlisted

Checks

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

┆Issue is synchronized with this Jira Task