[CI] support sending a PR to each ecs logging agent when spec changes

[v1v]

What is this PR doing?

It adds a Jenkins pipeline executing on pushes to the master branch, with the following stages:

• Checkouts the SCM
• Uses a parallel execution to send PRs to each subscribed ecs agents, if and only if there are changes in the spec files. To subscribe an agent, simply add an entry in the .ci/.jenkins-loggers.yml file:

---
loggers:
  - REPO: "ecs-logging-java"
    SPEC_FILEPATH: "ecs-logging-core/src/test/resources/spec/spec.json"

• Executes a shell script per agent, this script should:
  1. clone the agent repo
  2. ensure the target directory exists
  3. copy the spec.json file to the target directory
  4. create a git commit with the changes
  5. send a PR to the default branch of the agent repo with the changes

Why is important?

We want to push changes to downstream repos (the ecs agents) whenever a change on the specs is done. This way the repos are always up-to-date with the specs.

Issues

Closes https://github.com/elastic/ecs-logging/issues/30 Requires https://github.com/elastic/apm-pipeline-library/pull/936

[apmmachine]

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#### Build stats * Build Cause: `Started by timer` * * Start Time: 2021-02-05T04:03:00.653+0000 * Duration: 3 min 53 sec * Commit: 730c89ac5d57fd45d951c9edc5f4b7c462c5c58b

[axw]

Also, the two go repos appear to be using a different filename ("v1.json") rather than just using the same "spec.json" file basename. That means this code will need to adjust. Some possible ideas:

If it's less work overall, I think we could also just rename v1.json to spec.json in the Go repos. @simitt any concerns about that?

[v1v]

I just added support to different names with https://github.com/elastic/ecs-logging/pull/47/commits/c8205f56fdd5248f937f04e43d69216dfd41bab5 we can change if any repo changes the name of those spec.json files