add withCredentials support

[ViliusS]

What feature do you want to see added?

It would be very useful if this plugin would implement standard Credential Binging support via withCredentials. For now, if anyone needs to have Service Account JSON private key passed to let's say environment variable, you have to duplicate private key by uploading it to "Secret file" type. Then do:

withCredentials([file(credentialsId: 'privatekey-file', variable: 'GOOGLE_APPLICATION_CREDENTIALS')]) {
    sh "kubectl get pods"
}

So for Google Cloud and other Jenkins plugins "GoogleRobotPrivateKey" object is used, but for other custom stuff it's just a "Secret file".

Upstream changes

No response