[Addon] Allow enabling KEDA trait without installing the KEDA operator.

[madibaT]

Problem:

Ideally, it should be possible to enable the KEDA addon without installing keda operator on the cluster. Only the KubeVela definitions should be applied. This will allow users to use KEDA trait on clusters that has KEDA operator installed.

Proposed solution

vela addon enable keda --traitOnly=true

Considered alternatives

As a workaround, users can create the apply keda definitions only and that will work with the existing keda installation. The downside is that you will need to manually extract those from the addon as there is no option to only install the definitions when installing it.

Additional context

See this issue for the generic feature request applying to all the addons, not just Keda