Manage sealedsecret custom resource across clusters

[Wilt-James]

Which component: controller

Is your feature request related to a problem? Please describe. Our infrastructure team is preparing to move the sealedsecret controllers of each cluster to the same central cluster. Let the controllers of the central cluster manage the sealed secret custom resource of other clusters. But I found that currently the sealedsecret controller does not support managing the sealed secret custom resource of other clusters. InCluster configuration is used by default.

The relevant code is as follows

https://github.com/bitnami-labs/sealed-secrets/blob/0e3ca3f4b8d10299e24cf4dd932eea2afb2ee11c/pkg/controller/main.go#L139

Describe the solution you'd like Add a flag to set the cluster managed by the controller

https://github.com/bitnami-labs/sealed-secrets/blob/0e3ca3f4b8d10299e24cf4dd932eea2afb2ee11c/cmd/controller/main.go#L31-L31