feat: add config mode to datadog-setup.php

[realFlowControl]

Description

This PR will add a config mode to the datadog-setup.php, see PROF-6905

• adds an implementation of getopt() that allows for commands
• adds a config get, config set and config list command
• updates the help output

Readiness checklist

• (only for Members) Changelog has been added to the release document.
• Tests added for this feature/bug.

Reviewer checklist

• Appropriate labels assigned.
• Milestone is set.
• Changelog has been added to the release document. For community contributors the reviewer is in charge of this task.

[morrisonlevi]

I can't finish the review right now, but it's looking good so far!

[bwoebi]

I think the PR does a good job at handling all the possible cases :-)