[Feature] Add flag to skip installation when enabling addons about kubevela

[dhiguero]

Hi! I think this could be a nice feature for the type of use case you mention where FluxCD is already installed. I have used the alternative of rendering the components in the past as it is not the most scalable solution. I would suggest creating an issue specifically on the https://github.com/kubevela/catalog/issues repo referencing this one, and if you want to contribute PRs are more than welcome. I think that keeping this issue open here could be useful as we may think on generating a list of pre-defined options in the future for all addons, and onlyComponentDefinitions could be one of them.

[salvalcantara]

FYI @dhiguero, I opened this issue to target the fluxcd addon specifically first. Issues are cross-referenced now.