Installing Confluent Plugins on Kubernetes
To store custom connectors, use Nexus, Artifactory, S3, or some plain HTTP/file server.
If you are using Kubernetes, then you probably have a release policy around your Docker images.
Therefore, you can extend the Confluent Connect Docker images by adding additional RUN statements to the Dockerfile, then build and tag your images with Jenkins, and upgrade your Kubernetes services to use the new image tag.
The answer I would give for a bare-metal (or cloud) installation of managing Kafka Connect would be to use Ansible or other orchestration tool to push out the new files, and restart the services