Add dev tool for finding Jenkins jobs for a Beats PR

[ycombinator]

What does this PR do?

Adds a developer tool script, find_pr_jenkins_jobs.sh. This script will find and list out Jenkins CI jobs for a given Beats PR.

Sample usage

$ ./find_pr_jenkins_jobs.sh 15790
Searching last 100 Jenkins jobs for PR number: 15790...
https://beats-ci.elastic.co/job/elastic+beats+pull-request/8216/
https://beats-ci.elastic.co/job/elastic+beats+pull-request/8176/

Why is it important?

Could be useful when trying to find all old Jenkins CI builds for a PR, particularly if CI has been re-run repeatedly on that PR with a jenkins, test this command. This often happens on PRs that are testing solutions to flaky tests (e.g. #15790).

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
  - [ ] I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
  - [ ] I have added tests that prove my fix is effective or that my feature works