Add Support for Custom Certificates in Karmada Operator

[jabellard]

What would you like to be added:
Today, when creating a Karmada instance to be managed by the operator, it's not possible to use custom certificates for the instance. This feature request is for providing the ability to use custom certificates when creating a Karmada instance.
For reference, the helm chart already has support for this feature.

Why is this needed:
At Bloomberg, we're currently building a managed Karmada platform and want to use the Karmada operator to manage the entire lifecycle of managed Karmada instances. To fulfill some of our use cases, we would like to have the ability of specifying custom certificates when creating a Karmada instance.

[jabellard]

@RainbowMango , I'm working on a proposal for this feature request, and look forward to hearing your thoughts and feedback on it.