Add option and script to inject a custom apm-server into Elastic Agent

[jalvz]

What does this PR do?

This should enable us to run elastic-agent with an arbitrary apm-server binary.

Why is it important?

To run tests in pull requests with elastic agent and the apm-server from the current branch

Depends on #1016

[apmmachine]

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• Start Time: 2022-02-07T15:37:41.304+0000

• Duration: 5 min 47 sec

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[kuisathaverat]

this replaces the binary in the Elastic Agent container, if there is an APM Server running, would it restarted to pick the new binary?

[jalvz]

this replaces the binary in the Elastic Agent container, if there is an APM Server running, would it restarted to pick the new binary?

good question 😎

I didn't get to that part yet, I was waiting to have 1016 merged to see how it all plays together.
I'm going to move this back to draft in the meanwhile

[kuisathaverat]

I'd said that is easy to use the same approach we have for all the containers, build a derivated Docker container. In this particular case, a Docker container FROM the Elastic Agent container, then we add similar logic we have for opbeans to compile the APM Server and add it to the Elastic Agent Docker image.