Consolidate camunda-external-task-client-js into Platform

[venetrius]

Acceptance Criteria (Required on creation)

• Camunda platform contains camunda-external-task-client-js with its code and git history.
• XTaC NodeJS master is updated, source code removed, and README.md updated.
• XTaC NodeJS is integrated into the 7.X release process.

Links

parent issue: https://github.com/camunda/camunda-bpm-platform/issues/3682

Breakdown

### Pull Requests
- [ ] https://github.com/camunda/camunda-bpm-platform/pull/4316
- [ ] https://github.com/camunda/jenkins-job-dsl-seed-jobs/pull/416

[tasso94]

We decided not to incorporate the source code of the XTaC NodeJS into the mono repo. Here is why:

• Adding the npm publish to an existing freestyle release job like RELEASE-build-camunda-bpm-EE-tags makes it less reliable; if the npm registry has an outage, we are blocked from releasing Camunda 7.
• If the release job fails after the npm publish, we cannot retrigger the job since the release to the npm registry has already been pushed.
• Creating a dedicated freestyle release job allows us to retrigger the jobs independently of each other. However, the downside is that we are making our legacy release automation even more complex (we still use Jenkins freestyle jobs). This makes migrating off of this technical debt even more complex.
• Moving the source code of the XTaC NodeJS into the mono repo but not automating the release doesn't provide any value. Manual XTaC NodeJS releases are the only overhead we are currently suffering from. We don't have other positive side effects like dependency management.