Extracts ApmServerRule for use in Servlet integration tests

[codefromthecrypt]

What does this PR do?

Before we migrate to jupiter (e.g. via openrewrite), we should have code organized as typical JUnit 4 as possible. This extracts out static initialization in servlet ITs as a class rule while keeping all other logic the same.

Next steps will attempt to make TestApp more normalized, possibly via rules or nested or something similar, I'm not quite sure yet, but the goal is to be able to have tests be normal junit tests. Currently, they are loops which have complicated state because they need to peek back and clear apm logs between themselves.

Checklist

Left alone as this is test infra only

• [ ] This is an enhancement of existing features, or a new feature in existing plugins
  • [ ] I have updated CHANGELOG.asciidoc
  • [ ] I have added tests that prove my fix is effective or that my feature works
  • [ ] Added an API method or config option? Document in which version this will be introduced
  • [ ] I have made corresponding changes to the documentation
• [ ] This is a bugfix
  • [ ] I have updated CHANGELOG.asciidoc
  • [ ] I have added tests that would fail without this fix
• [ ] This is a new plugin
  • [ ] I have updated CHANGELOG.asciidoc
  • [ ] My code follows the style guidelines of this project
  • [ ] I have made corresponding changes to the documentation
  • [ ] I have added tests that prove my fix is effective or that my feature works
  • [ ] New and existing unit tests pass locally with my changes
  • [ ] I have updated supported-technologies.asciidoc
  • [ ] Added an API method or config option? Document in which version this will be introduced
  • [ ] Added an instrumentation plugin? Describe how you made sure that old, non-supported versions are not instrumented by accident.
• [ ] This is something else
  • [ ] I have updated CHANGELOG.asciidoc

[codefromthecrypt]

trying to figure out if this is a flake or not. locally so far so good

[codefromthecrypt]

abandoning this as there's some weird issue in jboss related to (no idea how) on this cd893419137fd80981505c754ecea5cce6fb0bbd