Support list of official managed OS channels for user selection

[aalves08]

Support list of official managed OS channels for user selection. For elemental-operator 1.6 and onwards.

Docs: https://elemental.docs.rancher.com/next/channels/#available-channels
Backend PR: rancher/elemental-operator#797 -> info will come from the Metadata resource
Backend PR: rancher/elemental-operator#800 (new PR)

Possible UX solution is to have another page, after hitting CREATE on list view, which would show the official channels + option to add a custom one.
If you clicked one one of the official ones, it would show the current create/edit interface with all the information pre-populated.
If clicked on custom, then it's the current create/edit interface without anything filled.

[anmazzotti]

@aalves08 The new backend PR is a new one: rancher/elemental-operator#800

[anmazzotti]

@aalves08 We are targeting elemental-operator 1.7.x for the channel enabled flag.
This won't make it on 1.6.x