Make api_key and secret_token dynamic

[JonasKunz]

What does this PR do?

Closes #2854.
Allows the api_key and secret_token configurations to be dynamic.
When the configuration option changes, all new connections to the APM server will be made with the changed key.
Existing connections will not be terminated on change, but they are already authorized anyway.

I did some manual testing by providing the secret_token via elasticapm.properties and changing it at runtime.

Checklist

• 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

[ghost]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-11-30T12:43:20.237+0000

• Duration: 68 min 50 sec

Test stats 🧪

Test	Results
Failed	0
Passed	3228
Skipped	40
Total	3268

💚 Flaky test report

Tests succeeded.

🤖 GitHub comments

Expand to view the GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run benchmark tests : Run the benchmark tests.

• run jdk compatibility tests : Run the JDK Compatibility tests.

• run integration tests : Run the Agent Integration tests.

• run end-to-end tests : Run the APM-ITs.

• run windows tests : Build & tests on windows.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[github-actions]

/test