[Proposal] Avoid communicate with Jenkins on DevOps credential modules

Current

The credential controller will create or update it to Jenkins via API calling once you create a DevOps credential via UI or CLI.

Potential Problems

• The extra cost of maintaining the credential controller in kubesphere
• Highly rely on Jenkins
• Potential exception from the communication from KubeSphere and Jenkins
• Users can modify (or remove) the credential from Jenkins side, then they might meet some confusing results due to they don't fully understand how does it work

Expected

• Less dependence on Jenkins
• No extra cost of unnecessary maintaining code lines

Solution

There's a plugin that can help us. kubernetes-credentials-provider-plugin can build a mapping between K8S secrets and Jenkins credentials. We don't need to maintain those credentials.

What do we need to do?

• Add some new mapping base on the extension points from kubernetes-credentials-provider-plugin. We can implement it in kubesphere-extension-plugin.
• Remove the credential controller from KubeSphere
• Remove an API from ks-apiserver (GET devops/{devops}/credentials/{credential}/usage)
• Remove other out-dated code lines

Desire reviewers

/cc @shaowenchen

Relevant Issues from Forum

https://kubesphere.com.cn/forum/d/3244-devops/19

/area devops
/kind feature-request
/priority medium

Start with pkg/controller/devopscredential/devopscredential_controller.go and the ks-apiserver GET devops/{devops}/credentials/{credential}/usage API. Review the extension points in kubernetes-credentials-provider-plugin and the proposed kubesphere-extension-plugin mapping. Done means the mapping replaces controller-managed Jenkins credentials, the controller and API are removed, and outdated code is cleaned up.