[JENKINS-61809] Include JCasC instructions in README

[orrc]

Since JCasC is now a thing, and it's not clearly mentioned somewhere how to configure a Google OAuth credential, and because somebody asked how it works, let's document it.

The config in the Jira ticket looked correct, based on what I could see from the PR which implemented JCasC support, and I was also able to get it working locally.

---

For info: if you want to try this out, create a jcasc.yml file with the contents that this PR adds to the README, and run Jenkins with JCasC, e.g.:

jcasc.yml

credentials:
  system:
    domainCredentials:
      - credentials:
          - googleRobotPrivateKey:
              projectId: 'Google Play'
              serviceAccountConfig:
                json:
                  secretJsonKey: 'eyJjbGllbnRfZW1haWwiOiJqZW5raW5z […]'

Dockerfile

FROM jenkins/jenkins:2.222.3

RUN /usr/local/bin/install-plugins.sh \
      configuration-as-code:1.40 \
      credentials:2.3.7 \
      google-play-android-publisher:3.0
ENV JAVA_OPTS -Djenkins.install.runSetupWizard=false

COPY jcasc.yml $JENKINS_HOME/jenkins.yaml

$ docker build . -t jenkins-61809
$ docker run --rm -t -i -p 8080:8080 jenkins-61809

You should then be able to use the Google credential in jobs to upload APKs etc..

You can also confirm that the credential was imported as expected via the Script Console, for example:

import com.cloudbees.plugins.credentials.*
import com.google.jenkins.plugins.credentials.oauth.*

def c = CredentialsProvider.lookupCredentials(GoogleRobotPrivateKeyCredentials.class)[0]
def config = c.getServiceAccountConfig()

println("Filename: " + config.filename)
println("JSON:\n" + new String(config.secretJsonKey.getPlainData(), "utf-8"))

[smellouk]

For future reference, this works well

import com.cloudbees.plugins.credentials.*
import com.google.jenkins.plugins.credentials.oauth.*

def c = CredentialsProvider.lookupCredentials(GoogleRobotPrivateKeyCredentials.class)[0]
def config = (JsonServiceAccountConfig) c.getServiceAccountConfig()
def method = config.getClass().getDeclaredMethod('getJsonKey');
method.setAccessible(true);
def result = method.invoke(config);

println("Filename: " + config.filename)
println("JSON:\n" + result.toPrettyString())